Hi again Nevada Girl,
Sorry about the confusion for you on our replies.
 The last explanation by Neemo is quite accurate.  Differing answers to your questions are based on differing levels of experience with regular data recovery used by most Repair Techs, and then there is a thing called Forensics Data Recovery, which I have been involved in.  This is sort of like the stuff you see on TV on crime shows such as CSI and NCIS (which is pretty accurate with lots of secret details left out).  This stuff is closer to rocket science than one might think, so it's unlikely you're ever going to fully understand it.  Having worked for 4 different hard drive manufacturers, designing disk drives and disk drive controllers, as well as teaching Computer Forensics Recovery to various law enforcement agencies, I am probably the most qualified person on this forum to answer your questions.  However, that doesn't mean you are going to believe me.  Additionally, I've held security clearances up to and including Top-Secret level clearances which means I can't explain all the details of how we perform Forensics Recovery on hard drives that have been "wiped", "erased", or intentionally physically damaged (such as the drilling of holes and sawing in half techniques mentioned).  Without disclosing any National Secrets, I can tell you that Forensics Recovery still allows data to be recovered even from these drives that have been intentionally destroyed.  In fact, that is how we found out who bombed the US Embassies in various bombings back in the 80s, long before 9-11 in 2001.  We also use secure erasure methods in various missile systems that employ storage devices as well.  I also worked on those.
With that information, it really doesn't matter about the details that you get, because we can't tell you those details (National Security issues again) even if we wanted to.  Jack was simply attempting to make it more easily understandable for you.  My comments were attempting to do the same.  However, my warnings were based on the fact that as far as law-enforcement and Government agencies and our Military go, they DO have the capability of accessing most if not all the information you mention you are trying to protect from a buyer of your used computer from getting.  Since you cannot be sure of whether or terrorist is going to be buying that computer from you or not, you have to err on the side of caution which it sounds like you are trying to do.
  That software you are using, is "iffy" at best, and since I am an expert and have never heard of it or used it, it's highly unlikely to be very good at all in this regard.  Since you are simply trying to safeguard private personal information on your Clients (dob, SS#, DL#, contact info such as phone number, address, E-mail, etc.), you don't have that much to worry about.  As long as your computer isn't used to commit a crime or a cybercrime (such as Hacking the Power or Water infrastructure facilities somewhere in the US), it's unlikely you have that kind of information residing on the hard drive of your home computer.  Or plans for building a suitcase nuke, or widespread deployment of a Biological Pathogen such as Anthrax), right?  So, if you are in the Banking, Real-Estate, or Stock-Trading markets for example, and are simply afraid that your Client information could be pilfered by a competitor in your field, again, the tools we mention here (along with the physical destruction methods such as hole-drilling and sawing-drive-in-half), software wiping should be Ok for your needs.
As Neemo points out, reinstallation of Windows itself or deletion of Restore Points and so forth are certainly 
NOT adequate protection with the recovery tools available on the Internet to anyone even without any expertise in Recovery to get data back from your computer's Hard Drive.   As Jack mentions in his Post here:
"go to your old computer … yank out the current hard-drive … bore (all the way) a dozen holes in that drive with an electric drill. better yet, take it to a metal shredder (probably $15-20) … watching them shred that paperweight into junk."
This is one adequate measure to use, that is the Metal Shredder recommendation.  Another way to go, is to simply 
REMOVE that hard drive from your Computer and put it in a 
SAFE place such as a Safety-Deposit Box at a Bank or a floor-safe or wall-safe in your home.  I like the Metal Shredder recommendation, because, if your Bank is robbed or your home is robbed either scenario could play out with that Hard Drive being stolen and going to who knows where for who-knows-what purpose along with cash, jewelry, bonds, etc.?
  If you were on Vacation out of town, state, or the Country, and came back to find that this happened to you, you'd never be able to get a good night's sleep again after reading my Post. 
For most of my Clients who have these same concerns, especially seniors who have read about stuff like this their entire adult lives, say 50 years or more, I give them a choice: let me wipe their drives with proper software tools and take to a hazardous waste disposal center that melts the drives down in a smelter and recovers their metals and plastic component materials, or I can wipe them and they can keep them under their mattresses, in-home safe, Safety-Deposit box in a bank *which many of them have*.  For the vast majority of them; 99% of them, they go with the 1st suggestion, relying on my expertise and explanation and reputation to make that decision.  The 1% or so, who are really paranoid and maybe served in a law-enforcement capacity, Government agency, Defense Contractor, or were in Military Intelligence went with the 2nd suggestion.  In other words, they took responsibility for keeping that drive for the rest of their lives, or they had access to places that could properly dispose of it while they watched it being done.  Those individuals never told me what they ultimately did with their drives even after me asking them years later.  Clearly they had sensitive information on them!  My job was done, as I gave them a clear explanation of the threat level, and consequences.  They then took the appropriate actions to protect that information as they saw fit.  Whenever a Client gives me their Hard Drive for wiping/erasure or destruction, I take that very 
SERIOUSLY as you can now see.  In fact, I am sure I take it more seriously than you do, as I'm responsible if information that is secure gets leaked out somehow.
This is probably way more information than you ever wanted to know about this subject, and there are many books that you can obtain about this at your local University library (I would not trust the Internet to read about such things) if you still question what I'm telling you.  I would opine to say that you are attempting to perform due diligence on your part by questioning what we tell you and that's very good on your part.
Hopefully, you'll find this information helpful in deciding your next steps about what to do with your computer and it's hard drive.
Very best of luck to you,
<<BBJ>>