One easy way to avoid that is just disconnect your main hard drive while doing this with the laptop drive, and have the laptop drive connected as master on the desktop.
This way, there will be no change whatsoever to your regular desktop hard drive. I'm assuming you have a bootable CD that you can use to add this bootloader.
Question: Are you interested in dual-booting XP and Fedora? Or are you going to wipe XP out altogether? There are many websites with details on how to dual-boot both of these OS's. It doesn't seem to be a problem, most of the time. But you should have them on different partitions.
You can easily fix your XP on the laptop by hooking up the laptop drive to the desktop pc, boot up with the Windows CD, then press "R" for recovery console. Once in Recovery Console, you would use either the "fixboot" or "fixmbr" commands at the prompt.
Here's a website that shows how to dual boot Fedora and XP, and resize the XP partition:
http://users.tkk.fi/~tkarvine/linux-windows-dual-boot-resizing-ntfs.html
You would want to resize the NTFS partition if your laptop only had one partition. Always best not to mix OS's on the same partition! If you already have two partitions on the laptop drive, disregard the resize part of the website above.
Edit: Mistakenly wrote "floppy drive" in my first couple of sentences, when I meant "laptop hard drive". I've corrected that above.