IF you have a installation key for WinXP AND you know what type of WinXP product you have you can borrow a CD of the same kind and copy it. Then use YOUR key to install. Post back if this explanation isn't clear to you.
BTW, it is perfectly legal to make backup copies of ALL your software. I ALWAYS do that AND install using the copy. That way you know the copy is good or not.
Burn copies of ALL your software now. CDRs are cheap.