To erase linux, just format the partition with ntfs or fat32. You can use xp's own tools for that. To uninstall grub or lilo from the bootsector, use winxp console recovery:
Boot the computer using the xp install cd. Then, choose recovery console, instead of new installation. Log into the installation. You'll be prompted for the admin passwd.
Once there, you can run the xp utility to write a boot sector, called fixboot, or fixmbr, something like this. Just run the tool, it'll write the new boot and overwrite grub or lilo.
Post again if you're still in doubt.