If your PC is SO OLD that it's still got Win 98 on it then you might be just as cheap to buy a new one.
http://tinyurl.com/dqzhq
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Failing that, the catch all solution is to install an operating system (possibly a newer one) straight from the CD and wipe your hard drive totally clean.
Another, more technical, solution is to identify the specific problem (could be one of tens of things) and fix just that problem. This can be very frustrating and time consuming even for a techie geek.
Just how much gumph do you have saved on your PC that 1) you need to keep and 2) you don't have backed up elsewhere (e.g. on CD) ??