Ouch. My father in law seems to be having a similar problem on his 98 pc.
Before reformatting and losing all data, 2 things. Disconnect any
unnecessary hardware (PCI cards, modem etc) and then see if it works better.
If so, add components one at a time. You may be able to pin it down on a
If this doesn't work, then I'd try getting a new hard drive and rebuilding
the system on that. If you can rebuild with no problems, then you may be
able to add your old hard drive as a slave drive and back up all info before
reformatting. This way you'll get more storage as well, or even be able to
start up form either drive in case of a future problem.
There is probably and easier way then the above two options, but that's what
I'd do. Maybe someone else has input.