Whew!! Problem with overloaded HDD, or... virus?

Lately my faithful old main rig, which I've kept virus-free for years without having to run anything other than Spybot S&D, has been acting quirky... just random slowness, etc. Recently Windows in its infinite wisdom alerted me that one of my drives (the one primarily for file storage) was getting full, so I dumped a lot of garbage out and made a note to start burning discs, esp. of the myriad photos I have on there.
well, today, almost at the very moment I decided "after this task (editing photos), I'm gonna get some of this off that drive", when I backed out of the main graphics folder (not the MS "My Pictures" folder- I keep my own filing system), when I went back to that folder, it seemed to be empty. 30-some gigs of files... poof. :o
I tried System Restore, forgetting I had it disabled...
After Windows finished its machinations and rebooted, however, my folder was full again.
So... as the title says- do I just have an old HDD that's freaking out because it's full (although I defrag about once a month), or do i have a virus in there?
well, today, almost at the very moment I decided "after this task (editing photos), I'm gonna get some of this off that drive", when I backed out of the main graphics folder (not the MS "My Pictures" folder- I keep my own filing system), when I went back to that folder, it seemed to be empty. 30-some gigs of files... poof. :o
I tried System Restore, forgetting I had it disabled...

So... as the title says- do I just have an old HDD that's freaking out because it's full (although I defrag about once a month), or do i have a virus in there?