This looks like a false positive: BitDefender is reporting Trojan.Generic.1423603 in C:\windows\system32\winlogon.exe. This name is sometimes used by malware, but in this case no other product is detecting anything malicious.
Current pattern is for BitDefender is 2640654, pushed out on Friday 13th February (!).
I will post the ThreatExpert prognosis when I get it.. in the mean time I would suggest that you do NOT try to remove winlogon.exe as you will render your system unbootable. (NOTE: Do NOT reboot your machine as this will most likely break it!)
Update: ThreatExpert indicates that it is clean. Several comments confirm that it is a false positive. The problem seems to be on Windows XP SP3, SP2 does not seem to have the same issue. The MD5 for this file is ed0ef0a136dec83df69f04118870003e
It seems that there are several reports at the BitDefender forum. I would guess that BitDefender are aware of the problem, temporarily disabling the anti-virus scanner may be a good idea else your system may become unusable. Usually these issues are fixed in 24 hours.
If you can't get the winlogon.exe out of quarantine, then this is a copy of the original (English US) file for XP SP3. Use at your own risk - password is "bitdefender".