Hey there -
I've been having problems with my computer (windows xp home), but I don't know if these popups i'm getting are actually real ones telling me to fix the computer (haha) or if i've been infected with something.. the messages popping up say this -
"Critical Error Message - Registry damaged and corrupted

to FIX this problem :
Open Internet Explorer and type : www.registrycleanerxp.com
Once you load the web pag, close this message window

After you install the cleaner program you will not receive any more reminders or pop-ups like this.

VISIT www.registrycleanerxp.com IMMEDIATELY"

---AND also getting this message---

"STOP! WINDOWS REQUIRES IMMEDIATE ATTENTION/
Windows has found 55 critical system errors.
To fix the errors please to the following:
1.Download registry update from: www.helpfixpc.com
2.Install Registry Update
3.Run Registry Update
4.Reboot your computer

FAILURE TO ACT NOW MAY LEAD TO SYSTEM FAILURE!"

Are you using XP2? If not, see info in link below for turning off Windows Messenger Service in XP.
http://itc.virginia.edu/desktop/docs/messagepopup/

If you have XP2 then follow the directions below.

Install Super Antispyware free. Run it in safe mode. Allow it to quarantine whatever it finds.
http://www.superantispyware.com/

Run the online scan for Bit Defender in normal mode. Allow it to quarantine whatever it finds.
http://www.bitdefender.com/scan8/ie.html

How to Start Windows in Safe Mode:
http://www.bleepingcomputer.com/tutorials/tutorial61.html