Need help finding '98 disk

Can someone help me find a Win 98 disk? I'm trying to help a long distance
sister who's system is crashing and I think the only thing we can do is
reinstall Win98, but she says she doesn't have a disk.
Anyone know how we can get one? Or in place of one what to do about corrupt
registry entries, and horrible startup probs? She's now got corrupt
antivirus program, registry, etc. I told her sounds like a virus but she says
it was ok on the scan yesterday. Thanks,ssteph.