So assuming that it is Windows 98, you'll know about the neat trick of CtrlAlt/Del to see the Programs running. Take it slowly and stop them one at a time taking note of the warning about two successive Ctrl/Alt/Del restarting the PC. If you do that you have to start over again.
NEVER stop Systray or Explorer, but everthing else can go, starting with your Anti-Virus. After each operation see if things improve and which application it was that did the trick.
Uninstall and re-install that one and it may do the trick.
However, if your set on a clean re-build don't let me put you off. That can be a very rewarding business and give your PC a new lease of life. And you get to jettison a lot of junk that you can't think of a reason for having.
I recently bought Norton Utilities for my Win2K system for a particlar purpose. It lasted two days because it clogged up the works something horrid and wouldn't co-habit with a couple of other Applications. I'll still use it when required but only for short periods and I'll put it back in the box after each use.