There are policies in the gpedit.msc in XP that you could set to prevent that: http://www.codinghorror.com/blog/archives/000294.html
Specifically option 2. I'm not trying to be a dick but I find it hard to believe that you couldn't have found that yourself in 15 seconds on Google.
Funny, I always nag about Linux eating my Windows bootloader 
I complained about the reboot problem IN VISTA. Not XP.
And why in the world would you want the Windows bootloader?
