If you do not have a floppy boot disk for windows download UBCD to boot your box back up.
Do you have a full MS XP CD or is it an OEM "rescue" disc?
You may also have to go into the bios to change the boot order from:
A:, C:, D:
to D: A: C:
The D: drive is typically the Cd-rom drive. Don't panic if the CD-rom has a different letter just change the boot order to the appropriate drive letter and the MS disc should work.
Be advised that you may have call Microsoft to re-activate your registry key.
Do Not tell them that you nuked XP for a linux install. Tell them that you got zapped by a virus and had to do full clean install to eliminate malware.
you're going to need it.