I'd get rid of both 98 and XP and install Win2K, if you can get it. (I can always send you a copy.) It's much more stable, IMO. I have all three on this computer, but prefer to use 2K for this reason. It doesn't fragment nearly as much as 98.
BTW, I was always under the impression that SCSI was faster than IDE, depending on type. He may not have a choice but to keep it, if he still wants to use that scanner. Check your Device Manager (right-click My Computer, choose Properties, then Hardware) and make sure there aren't any conflicts and that everything is working properly. You'll know, because you'll see yellow exclamation points or red x's. If you see that, let us know, and we can try to tell you what to do.