FYI
oddly enough, another tech and I worked on a computer with the same problem the day after you posted this thread. We worked on it yesterday doing any and every thing we could think of, including reloading the chipset drivers, keyboard drivers, manualy editing hardware related reg entries, replaceing and registering winlogon and GINA executables and assciated dlls. We ran a CHKHDSK /f, used a thrid party error scanning software, attempted a XP repair from the CD, amongst other stuff.
In the end, for the sake of time, I re-imaged that PC which essentialy wiped and reloaded everything on the HDD. Problem solved, but it wasn't hardware problem.
If you're up for it, you can try all of that. I used another computer to remotely connect and work on the PC. You may also check if there's any additional client software on the compuer, like Novell, that could be causeing it to lock whne you hit the CRTL+ALT_DEL, or you can also try to upgrade the bios. I'm not sure, but there is a possibility this is a hardware problem.
hell, it may just be easier to setup xp on another HDD.
Hope this helps
Good Luck!
Chris