No, boot from your XP CD, enter R at the appropiate time to open the Recovery Console and, after entering your password, enter
fixmbr at the prompt and confirm that you want to rewrite the MBR. Reboot into XP (without the CD in, of course). open Control Panel -> Administrative Tools -> Computer Management -> Disk Management, then delete the Mandriva partitions.
P.S.
Then install Slackware.