Ok, this is a guide, for if you want to install back Windows XP but keep your current Mandrake installation.
What do you need?
Windows XP installation CD :p
Mandrake Installation CD
Eventually a live cd Like Mandrake move
Diskdrake(or any similar partition program)
Without Live CD
You don't need the live cd, but it depends on which partition you want to resize for making free space for XP. I resized my /home partition, without using live cd. Launch a terminal, and go to root mode, and type
This ends your graphical session, and goes to console.
Make sure your logged in as a root, and unmount the partition you want to resize( in this case /home)
Now you have some free unpartioned space, which you can use to install XP.
Reboot your computer and insert Windows XP cd, to start the installation. When it asks you for partition, create a partition in the free space(NTFS!(other partitions will be marked unknown). It says something about XP & dualbooting, but that are worry's for later, just install XP like you always do :).
When installation is finished don't be scared, cause it will boot XP auto(your lilo is gone, thats why you need Mandrake installation cd).
When everything is configged in Windows, reboot your computer with your Mandrake installation cd inserted. When it boots, press F1 for more choises, type
now it will launch a ncursed? screen, where you can restore your lilo config.
When you reboot, it will launch mandrake again, and then you just have to add Windows XP in your bootloader config(manual or through Drakxboot).
Now, you have a working dual boot system
Please use this guide at your own risk, report mistakes to me, so I can correct them, make sure if you don't trust it, that you have backups