WinXP's 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, it'll look something like this:

ExtraData=707A667567736F696F697911AE7E05
Pid=55034000

The Pid value is what we're interested in. What's there now looks like a standard default. There are special numbers that determine if it's 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 you'd 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, you'd use:

Pid=51882OEM

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