I have done this with my T61p. The stock XP Pro that came with it was bloated so I made my own custom XP install disc.
You can use any XP Pro disc and change it to your OEM product key later. I used a corporate XP Pro and used Microsoft's product key changer to change it to my OEM license.
I simply copied all the files off the disc and used nLite (
http://www.nliteos.com/), driver packs from
http://driverpacks.net, and RyanVM's XP post-SP2 update pack (
http://www.ryanvm.net/msfn/). Start with the post-SP2 update pack (download the pack and use the RyanVM integrator). This will modify .cab files on the disc and slipstream all hotfixes up to the date of the update pack. Then download DriverPacks BASE (
http://driverpacks.net/DriverPacks/download.php?pag=b) to slipstream drivers and get packs for it here:
http://driverpacks.net/DriverPacks/. I learned the hard way that you must integrate SATA drivers otherwise the XP installer won't detect your SATA hard drive! I slipstreamed chipset, LAN, mass storage, WLAN, and sound drivers to my disc. Note that sound drivers will only slipstream if you already slipstreamed RyanVM's update pack and that is why you should do that first.
This part is optional, but you could use nLite to trim off the bloat that comes with XP and adjust it to your taste on first boot. You could even make an unattended XP disc if you provide all the necessary info and your license key.
nLite is useful for making bootable .iso files. Just burn a bootable CD (I used Alcohol 120 to burn) and boot from CD to install.
After installing, my custom XP Pro had wifi, sound, and lan working on first boot. You will need to install ThinkPad-specific software manually though. I didn't slipstream graphics drivers because I opted to use beta nVidia drivers elsewhere.
I hope that helps. cheers