Well the second antivirus you installed contained malware.

A friendly tip, in the future do a little research on your antivirus first. I recommend Avast! Antivirus, as they have a version free to non-commercial use (not for businesses but free for personal use) and it's updated frequently as well as being completely free.

Data Execution Prevention (DEP) is a set of hardware and software technologies that perform additional checks on memory to help prevent malicious code from running on a system. In Microsoft Windows XP Service Pack 2 (SP2).

I would try starting your computer up in Safe Mode, then remove the software you installed.

If you can't find the software, or are having continued problems, feel free to click my name above this post, and send me a private message.

I have a similar problem, and browsing through forums found this one the closest to mine.
I installed Avast to clean Antivirus XP Pro and it worked, then I installed COMODO fire wall.
On the next restart Comodo asked me to remove/quaranten bunch of files. I did almost all of them (now fearing that I might have deleted something useful) Files I deleted via Comodo interface were mostly .exe files such as googleupdater, and so on.

On the next restart Windows XP goes into log in mode (which never did before) and if I click on a user name of admin name it immediately displays the message "loggin off" and "saving preferences" and then I have a log in screen right away. I can not brake this endless cirlce past the log in screen.

* Safe mode gives me the same options,
* logging in as admin revers me back to logging in again
* CTRL+ALT+DEL(twice) brings me from this welcome screen to a log in screen and if I attempt to login it goes back to welcome screen

Any advices?