First: HAVE YOUR WIN98 CD HANDY! You may also need the CD or disc that came with your motherboard.
Then, go into Device Manager and delete the IDE controller devices. You'll be asked to reboot, and then they will be detected and the drivers reloaded. You'll reboot again. After the second reboot, all should be well. If not, you likely have some kind of hardware problem.
One thing that commonly happens that gives lots of strange symptoms is the power supply dying. Most PC power supplies are cheap crap, and will wear out, with the voltage and current dropping slowly over time until it can no longer put out enough juice to run everything. When that happens, whatever components are the most sensitive to voltage variations will start acting up first. Anyway, it's something to consider.
-Troy