If you're running one of the versions of windows, check how much free space you have on your hard disk. If the free space is getting really low, this might be causing problems. Windows uses some of the free space on your hard disk to temporarily store things it's working on (not just documents, but system data it uses to manage itself while it runs). if the disk is close to completely full, have a look for junk on the disk that you don't really need any more and delete it.
Also consider defragmenting the hard disk once in a while (I only do it 2 or 3 times a year, but I try to always keep 1 gigabyte of free space on my 12 gig disk). As you create and delete files on the disk, they get spread across it so you end up with lots of small chunks of free space scattered all over the disk (this is fragmentation). The computer will run more efficiently if all those little chunks of free space are merged into one great big chunk of free space (this is defragmentation).
If you are running one of the windows versions, they all have a defragmentation program built into them, but it will need a certain amount of free space to work properly as it moves files around to merge all the small chunks of free space back together again. If your disk is really full, you'll need to delete some stuff just to give the defrag program enough space to work with properly.
If you have pleanty of free space on your disk, I have no idea what else might be the problem.