When you repair the OS it won't ask you to enter in a Product ID (Key). Only when you do a fresh installation of XP will it ask for a Product ID, because a key was already entered in the first installation.
If your OS and his are both Windows Professional XP (with or without service pack 1 or 2 it doesn't matter), then yes you can use your legitimate Product ID to validate his copy of Windows. You can change the Windows XP Product ID in Windows the following way...
Open this guide: http://www.petri.co.il/change_the_se...windows_xp.htm
1. Complete steps 1 to 5 in one of the guides above.
2. Go to Start
3. Type x:\Windows\system32\oobe\msoobe.exe /a
, where x:\
is the drive where Windows is installed.
4. Click Yes, I want to telephone a customer service representative to activate Windows
...but don't continue with the process. Instead...
5. Click Change Product Key
6. Enter the new Product ID.
This is perfectly legal. Attempting to validate your machine with an illegitimate Product ID, or attempting to circumvent the process is illegal.
An Activate Windows
box should appear. Click to change the Product ID. Try entering the new key using the internet connection first (assuming a connection is present).