You want to do WHAT?? Remove Linux, keep WinXP??
IMHO, that's CRAZY!!
You could do this about any you want to (fdisk from linux, fdisk from windows, partitionmagic, etc.), but here's how to set the boot flag for windows partition from a linux console:
Mandrake probably has some GUI tool, but fdisk (a console utility) is pretty easy to use:
For my system, I'd type (stuff in " marks)
1. "fdisk /dev/hda" since I only have one IDE hard drive
2. "p" to make sure you know which patrition is WinXP
3. "a" to make a partition bootable
4. the # of the WinXP partition (you should know from step 2)
5. you can also delete partitions, create new ones, etc. but i'd recommend going into windows or partitionmagic for that since you are still running linux off of the partitions that you want to delete
6. "w" to save and exit
If you get confused you can use 'm' for help or 'q' to just quit without writing anything.
Reboot, WinXP will load and you can do the rest of the damage - I mean removal - from there. A program also called fdisk exists for Windows too (although different). You can delete the linux partition(s) and create whatever you like in their place, then format them.
Word of caution: if you want to resize your WinXP partition, don't do it while running WinXP, instead use a boot medium.
Hope this helps.