Well, that's a tough one unless you're willing to setup some physical access restrictions such as install a lock on the door to the room where your PC is. One trick I used to recommend to customers with computer savvy kids, is if you can't lock the door to the room the PC is in, such as a living room or a study with no door; buy yourself a locking 2-drawer or 4-drawer STEEL file cabinet $75-$125 or so, and
disconnect the keyboard from the PC when you leave your house to run errands or go to work. You kid will not be able to login to your PC without having a keyboard connected with which to type in the password or utilize hacking tools on! This works really well. Remember to not leave the file cabinet drawer key laying around where he can find it; best to put it on your key-ring so only you have it.
No keyboard--no access to your PC!
When you get home and want to use your PC, unlock your file cabinet, remove your keyboard and plug in to your PC--you're ready to work! When done, unplug keyboard place back in locking file cabinet and your protected!
Oh another thing, I recommend you take your PC to your local licensed Computer Pro and have them clean off all viruses and Hacking Tools. There is a Hacking Tool called a
keylogger, which your kid probably find out about from other school chums or hacking sites. The Hacking Tool could have been installed on your PC when you weren't watching him closely; it only takes a few minutes. Then the next time you are logged into the computer and you let your kid on to do his homework, he launches the
keylogger file and opens up the program which has captured your Windows login password to a text file.

He can then read your password in the text file, copy it down, and have it whenever you're not home to login to your computer!

It's also pretty easy to print out the keylogger capture file with the password to a connected printer which most folks have on their computers at home.
**THE NASTY PART ABOUT THIS IS, IF YOU DECIDED TO CHANGE YOUR PASSWORD TO KEEP HIM OUT OF YOUR PC; THE NEXT TIME YOU LOGIN HIM IN AND TURN YOUR BACK--THEY CAN OPEN UP THAT KEYLOGGER PROGRAM AND GET YOUR NEW PASSWORD AND GET RIGHT BACK IN AGAIN! SO, EVEN IF YOU CHANGE YOUR PASSWORD EVERY SINGLE DAY, UNLESS YOU GET THAT KEYLOGGER REMOVED FROM YOUR PC, YOUR SON CAN GET INTO YOUR PC WHENEVER YOU'RE GONE FROM HOME!!!***
And, when you do take your PC into your local Computer Pro to service; make sure you tell him that you suspect he/she has been installing and using Hacking Tools, including a possible
keylogger onto your PC to gain unauthorized access to your computer when you're not home. This is important; as these programs often can't be removed with standard program removal methods. It takes expert level skill to remove them. Many Techs who are not properly licensed, do not have the education or skill to do so. If you get a Tech who doesn't know how to do this, they will probably tell you they cannot find any
keylogger programs and will offer to do a Windows Reinstall. You should tell him that's what you want. It will cost you even more money than the Service Call for a virus cleanup; but worth it as that process will definitely remove the
keylogger from your PC.
Remember that Physical Access is the best way to keep your kids out of your PC! But, nothing substitutes for good parenting and supervision. I have Customers who use their Computers for babysitters, and give them a laptop and let them go into their rooms for 2/3 days and even lock their doors and the Parents have no idea what they are doing with that computer. I see those often, and the parents when I tell them not to do that, nod their heads in agreement, and then a few months later I get a call from the same Parent asking me to once again clean up the mess on their laptop.
Hope that provides you with some good information,
<<<<BIGBEARJEDI>>>>