Microsoft has stand alone memory diagnostics, and at least some of the older Norton Utilities had good diagnostics. I believe Windows 98 had a System File Checker, Seagate and Western Digital have good hard drive diagnostics. You should also boot to Safe Mode and check for viruses.
I have to say though that if you're running Windows 98, it's time to upgrade to Windows 7, most likely on a new PC. Windows 98 has been unsupported both by Microsoft and most applications and utilities so long that you're just going to run into more and more problems if you continue to use it.