Reinstalling windows on a dual boot system

[sid]

Hi
I have Windows Xp prof and Fedora 3 installed on my laptop. I need to reinstall my windows xp cos 
its showing its native behaviour..like system hang.. Now i read that if I reinstall windows..it will 
overwrite my current boot loader(GRUB)..and then i will not be able to boot linux. Is there a way to 
do it without losing my linux installation??
Also has any one tried to work on Fedora installed on a usb hard drive? how is the performance? I am 
considering buying a usb hard disk for working on linux on my laptop.
thanks in advance..
Sid