Are Replacement OS disks available?
 

I am trying to help a friend, but they can not find their Windows XP OS disk.
Can I get a replacement disk, if I have the product key code?

Yes, many manufacturers will offer to sell you a system disc. You can check on their support section of the manufacturers website.

Also if you have a valid key that is linked with the machine (eg OEM) most time you can install the OS from any disc that is similar,
OEM XP Home SP1 with OEM XP Home SP1,
Retail XP Pro SP2 with Retail XP Pro SP2.
The license should work as long as it's valid and your using it on the same machine it was originaly registered too if it's OEM (came pre-installed).

If the mfg. offers a replacement disc, I would suggest you go that route. It will be easier and work for sure.