i'm wanting to install Windows XP on my Fedora 10 PC so they run in dual boot mode... the reason i want to use USB is because:

1. I have a 4GB USB Stick
2. I have an ISO slipstreamed with SP3 and other necessary softwares (Avast/7zip/Winamp etc...) [hate the update downloads)

Now my main concern is: I know windows will alter the mbr so i'm wondering if its difficult to reset the mbr to be handled by Fedora bootloader?

Or what are the risks on installing Windows XP into a Drive with Fedora already in it... i have unused space on the drive which windows will slot into

If you want specific advice about your particular situation, I would suggest you boot into Fedora and log in as root to run the command: fdisk -l (lower case Letter L) and post the output here. This command shows the partition information for your computer. It's not difficult to re-install Grub and there are many posts here at LQ explaining it.

One thing it will do is overwrite your mbr, won't ask or tell you it's being done. But that's easily overcome re-installing Grub. Make sure you know which partitions have Fedora on before you do the install of xp so you don't write to the wrong partition.