How can I tell if it's my hard drive or something else?

I'm having a lot of problems with programs freezing, or Windows not opening, or not closing all the way. Even Safe Mode won't boot up at times until after several restarts, and when Safe Mode is up and running, it's running very sluggishly, like taking hours to do a defrag. I'm running XP on an older computer that never used to have these problems. If you have some advice let me know how to check what the trouble might be.

Download/install http://crystalmark.info/software/CrystalDiskInfo/index-e.html. If it shows Disk Status=Warn or Bad, it's time to replace it. If Status=Good then disk is OK

Thanks. I did as you suggested and it came out as "good". So I guess there must be something else wrong. Now I need to know how to diagnose that problem

That says the physical disk is OK. I'd run chkdsk /r next to verify the disk filesystem and repair any issues next. http://support.microsoft.com/kb/315265