If you installed Mandrake, I'm guessing your bootloader is LILO. The choices you get when you boot up are determined by the configuration file for LILO, which should be named lilo.conf. I can't recall exactly where that file lives. It's probably in /etc. If it's not in /etc, run this command to find it:
find / -name lilo.conf -print
Now, if you have just one hard drive in your machine, it is probably designated hda, and different partitions of it will be hda1, hda2, etc. From your description below, it sounds like you have three partitions, one with XP installed, one with Mandrake, and one idle, so I guess you have an hda1, hda2 and hda3. If you don't know what partition your XP is on, here's one way to find out that I can think of off the top of my head. Use cfdisk to see the partitions on the disk:
cfdisk /dev/hda
You will have to be root. The partitions and what sort of filesytem is on them will be shown in a table. It should be obvious which is the windows partition. Take note of the partition name and quit cfdisk. For illustration purposes, let's pretend it is hda1.
Now, open your lilo.conf file with a text editor like vim or gedit. Go to the bottom of the file and add two lines like this:
other=/dev/hda1
label=winXP
Remember, we picked hda1 for illustrative purposes, if the XP partition is different, change the line accordingly. Also, the label can be pretty much whatever you want--it probably should not contain spaces, and there's almost certainly a limit on how many characters, but you know what I mean. Okay, save lilo.conf and exit your text editor. Now you run the lilo command:
lilo
And you'll see some output. If all of the above worked, you should be able to reboot and lilo will present you with the option to boot into "winXP" or whatever string you choose for your label.
Jough, anyone, can I get a peer-review of these instructions?