No. Since you are trying to setup the RAID after XP is already on a disc you will lose that install if you setup RAID now.
Sorry scratch that. Let me explain fully.
Yes it could work but ONLY if you set aside partitons on EACH drive of the same size and leave XP alone on its own partition not apart of the RAID.
That way you can setup the RAID with drives of the same size which is required and still have your XP Partition setup on non RAID.
IF you had to use teh drive space that XP is on and make the whole drive RAID then no. Since that area will be technically only 1/2 of teh actual drive and the parts that would be needed wouldnt be located on the other part of the RAID where they should be then XP wouldnt boot.
I hope that makes some sense.