Im about to do a repair on a system with a fluffed windows install.
They dont have any XP CDs just a recovery partition that will destroy the world.  I have an XP Home CD, but its not an OEM copy like the one for their system will have been.  

They have a COA for Home, branded "GATEWAY"
What do I need to change the PID to in the setupp file?

Log in as the local Administrator
Click Start --> Run --> and type in Regedit
Browse to: HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\wpaevents

In the right pane, right-click OOBETimer, and then click Modify
Change at least one digit of this value to deactivate Windows
Click OK and close regedit
Click Start --> Run and type in: "%systemroot%\system32\oobe\msoobe.exe /a"

Click "Yes, I want to telephone a customer service representative to activate Windows, and then click Next

Click Change Product Key (at the bottom)
Enter your valid Corporate Product Key
Press Update and close the window.
If you are returned to the previous window, click Remind me later
Restart your computer

But I do not know if it is just the pro/corp editions or if it works on home versions as well

Close, but I think Ive found it.
On the Windows XP CD, change the PID in the setupp.ini file to OEM at the end.  Then it will accept OEM keys.

Nice one soopah, been wondering this for a while for my laptop

 WinXPs setupp.ini controls how the CD acts. IE is it an OEM version or retail? First, find your setupp.ini file in the i386 directory on your WinXP CD. Open it up, itll look something like this:

ExtraData=707A667567736F696F697911AE7E05
Pid=55034000

The Pid value is what were interested in. Whats there now looks like a standard default. There are special numbers that determine if its a retail, oem, or volume license edition. First, we break down that number into two parts. The first five digits determines how the CD will behave, ie is it a retail cd that lets you clean install or upgrade, or an oem cd that only lets you perform a clean install? The last three digits determines what CD key it will accept. You are able to mix and match these values. For example you could make a WinXP cd that acted like a retail cd, yet accepted OEM keys.

Now, for the actual values. Remember the first and last values are interchangable, but usually youd keep them as a pair:

Retail = 51882335
Volume License = 51883 270
OEM = 82503 OEM

So if you wanted a retail CD that took retail keys, the last line of your setupp.ini file would read:

Pid=51882335

And if you wanted a retail CD that took OEM keys, youd use:

Pid=51882OEM

Note that this does NOT get rid of WinXPs activation. Changing the Pid to a Volume License will not bypass activation. You must have a volume license (corporate) key to do so.