Uh-oh. "Operating System Not Found" usually is an omen about the health of your hard disk. You get that message if the BIOS can't find the hard drive or the master boot record is corrupt, etc. There could be a number of reasons for that - the hard drive (IDE) cable might be loose, the power supply cable to the hard drive could have a short, or your disk might be going bad. It could also be a virus.
If it looks like all of the cables are in place and look unmolested, then I would be concerned about the hard disk going bad. My suggestion, provided you want to put money into it AND you are certain you don't want to lose your important files, would be to either a). go buy another internal hard drive and switch it out with yours or b). buy an external hard drive that connects via usb and hook it up and copy your important files over to it.
a). will solve the problem if it's a bad hard disk, but that's about it. If there's a virus on the disk, you'd have to reformat and lose your important files to get rid of it.
b). is what I would do in your situation. First, you get to save your important files, so you can reformat your internal hard drive and reinstall 98 (from the system recovery cd like Y2 said) without losing anything. Then, if it turns out that it really is a bad hard drive, you can get another internal HD and still have those files. You'll always have the external HD to keep a copy of important files on too.
Of course b). will be more expensive. A standard 40Gb hard disk nowadays can go for right at $100 and an external hard disk for about $200. And you still would have to buy another internal disk if yours is bad.....