Uh yah, from 98 to XP is not really a "smooth" process, it's better just to reformat and get a real version of XP.
It's also illegal, nice going, hope the feds don't troll this site too often.
If the PC was mine, I'd reformat it, and do a fresh install of XP. This ill get ri of any possible problems that you had during the upgrade process, even if you weren't aware of any. There are too many differences.
EX: Windows 9x uses a FAT16 or FAT32 format on your harddrive, this is somewhat archaic today and has a very unorganized/wastefull storage file system structure.
Windows XP uses NTFS (along with a newer version of NTFS), slightly newer and alot more organized, better security, etc.
You can't simply switch from a FAT partition to an NTFS one. I know the "upgrade" makes it seem like you can, but it's definetly not recommended even my Microsoft. Format in NTFS and install XP.
Also, when installing a new OS, never be overclocked/tweaked in anyway. This can really screw up th install. Wait until it' all said and done, then overclock again.
EDIT: Google.com is nice. - http://www.majorgeeks.com/Prime95_d4363.html