I reformatted  New Year's day (running XP Home) and my machine was running great until about a month AGO, at which point it began slowing down and acting buggy.  It has gotten progressively worse since then.  Last night, I got an error message stating that the disk (the C: drive, that is) was nearly full, which surprised me, as it is an 80 GB drive, and to the best of my knowledge has only 35 GB or so on it.  I unhid the system folders and checked properties for the "size on disk" of each folder.  They totaled about what I thought, 35 GB.  I RAN cleandisk (or whatever it's called) and freed up a few stray MBs.  Then I compressed the files on the disc, after which the disc's properties claimed it had some 58 GB on it.  The machine is running much better now, but it STILL thinks it has 20 or so GBs more on it (all these numbers are from memory since I'm at "work") then it really does, which not only means I have some 20 GBs I can't use, but perhaps more disturbingly, means either that Bill Gates's idiot SON Windows can't properly read the disc, or the disc is going batty.  Both possibilities trouble me.  Any thoughts?You can look at the space the system restore is using.

Also look at how big your swapfile is....

I'll check System Restore (I'm a little embarrassed that I didn't think of that).  But how do I check the swap file?  Never heard of it...