Programs are suddenly unable to acces their (virtual) installation directory

I've been using Vista Home Premium x64 for about 3/4 year now, and - until today - never experienced any major problems. The most annoying bug so far was Vista not remembering my folder settings, but thanks to these forums, that problem been solved (for now) .

Anyway, today a much bigger problem came up. My programs are suddenly unable to create/modify files in their installation directories (e.g. C:\program files (x86)\[program name]\...). I know that this seems to be correct, since Vista should "redirect" the write commands to the 'virtualstore'-folder.

The problem is, though, that this "redirection" doesn't seem to take place anymore. Programs either crash or display error messages (e.g. "unable to acces [filename]") whenever they want to create or modify a file in their install dirs.

As a result, I am unable to (fully) use a lot, maybe even most, of my programs.

Using System Restore didn't solve the problem, so I'm hoping anybody here can help me. I'd really appreciate it, thanks in advance .

BTW: the problem first occured after installing pdf printer software - don't know if that's a coïncidence or not, though. (I've already uninstalled the software, that didn't help)

Edit: I am able to modify and create files in the virtualstore folder and its subfolders. Also, Notepad was allowed to save a .txt file to a subfolder of virtualstore, so the folder itself doesn't appear to be the problem.