Do you have a Windows PE bootdisk? If not, go to http://www.nu2.nu/pebuilder/ and download the BartPE builder and follow the instructions. Just a basic WinPE boot disc will allow you to do whatever you need to do to recover. Whether that be replace minlogon with the original Winlogon, or revert the registry change (which key did you change anyway?).
BartPE is a free utility that will use your existing WindowsXP or Server2003 install files to build a Windows PE-like recovery disc. I believe you can also get it to load onto a USB thumbdrive and boot from that, however unless things have changed from a couple years ago, it was both slow and required some 'trickery' in order to work decent.