Are the internet problems on XP or 98? If they're on 98, make sure it has drivers for your hardware by downloading them onto your XP drive or the family computer and transferring them across via CD or floppy disk. I presume you're using NTFS for XP so you won't be able to access it directly from 98 without NTFS drivers. USB will be a problem in 98 too.

If the trouble is on XP, in all honesty, I'd just format and reinstall Windows. It's the most painless solution for me. If you back up the stuff you want to keep you can restore it fairly quickly.

Now that I mention restoration, have you tried a system restore point?