STOP c00002a (fatal system error)

This is most likely caused by a bad GINA-dll messing up the winlogon.exe.
To fix this; simply boot to safe-mode, open regedit and drill down to HKLM\SOFTWARE\Microsoft\CurrentVersion\Winlogon and note the value of "GinaDLL".
In my case the GINA were pssogina.dll, the PointSec Single Sign On component. The STOP-message appeared after updating Thinkpad Access Connections to 4.41 and selecting to install the Access Connections-GINA.
I booted to safe-mode, ran regedit.exe and altered GinaDLL to "msgina.dll". The following boot loaded XP successfully, now with the default GINA.