if you can boot into safe mode then you're definitely a recoverable scenario. I highly recommend a little program called HijackThis v1.9 (whatever is most recent).
Download.com it doesn't need to be installed, and on starting it you want to choose do a system scan and save the log. Posting the log info will help dramatically. This gives a description of everything running on your pc and what's attached to critical components like winlogon. Now if you can get into windows normally this is the better place to run the hijackThis log as in safemode you load different registry settings than normal mode. On a quick note, winlogon is very very often attacked by/attached to by viruses and spyware as it is garaunteed to always be a running program on any windows pc. And on a similar topic, many viruses and spyware now are smart enough to infect/attach to the system restore directory backups that XP does on a regular basis. This is where a bootrom like MiniPE/BartPE really comes in handy as you can load into a stable environment and access your registry files as well as registry backups quite easily.