Wow, someone else with the same problem!
Something very similar just happened to me, although mine isn't as extreme as yours. I do have access to my documents, but little else - access denied.
I was able to determine that somehow my only account has been switched to a Guest Account and I haven't found a way around it.
I'm using the x64, Homegroup and it happened in the last several days. I posted in the TechNet forums the following:
Completely baffled as I didn't experience this with the beta.
Installed RC1 as a HomeGroup, only 1 PC in the group, only one user. I went to migrate some templates from my beta x86 folder backup and paste them into my RC1 x86 folder. "You need permission from an Administrator to do this". I checked my Account type and it says GUEST??? I selected Administrator but it won't take.
I tried
accessing the Administrator account but don't have rights to even make it show up in the User Accounts.
I also tried creating a new account but it kept saying "The specified account name is invalid. It must not contain ;:'/ blah blah" but the name is 123 or Robert or...
Just determined that it was not this way for about the last month, this is something that happened within the last few days. The only change I can think of is that I changed the login to not require a password. I'd change it back to requiring a password but it WON'T SAVE THE CHANGE! GRRRR.