Something you can try, if you have not already - this sounds like a permissions issue, virus or whatever reset permissions so that you're denied access. 
 
Right click on start, select explore, you'll see a list of of folders, right click on the program files folder and select security, you'll see a list of users, select the user that is you ands make sure that full control is checked, then hit the advanced tab and check the propogate to child objects is checked so that will give full control to all folders under program files.  If you don't have full control then this virus or whatever is changing permissions, also check your other folders.   
 
Anyway just something to check, hard to troubleshoot things via message boards.