overwriting hard drive with 98 disk

[trainwizard]

I have a pc running vista and want to downgrade to xp but I only have the xp upgrade disk. I tried to install my 98 disk onto it to upgrade to xp but 98 says "do you want to replace current OS" I said "yes" but 98 says that "your computer may have a disk compression program which may contain an operating system" setup is then canceled. So I guess I need to know the best and cheapest way to wipe vista off.

[Disk_Contented]

You need to clear the master boot record, etc.
Try the setup boot disks from M$: http://support.microsoft.com/kb/310994
See how you go. At least they should enable a clean start.
Otherwise obtain a 98 boot disk and proceed from there. (fdisk.exe /mbr) http://support.microsoft.com/kb/69013
Be aware that your HDD may not be properly recognised by 98.