Last night I looked at my taskmaster, only t'realize that my Winlogon.exe is taking up *256M* of my memory.. more than half my systems memory.

This morning, I reboot, and check.. Its back to under a meg of memory.. Now.. 12 hours later, its back up to 128M of Memory being used.

Anyone have any idea what's goin' on here and how to stop it?

Which version of Windows are you running?

The original Winlogon.exe from Microsoft gets placed in the C:\WINDOWS\System32\ directory. If you find it anywhere else then you should be suspicious for sure. Quite a few viruses hide under that name:
http://search.symantec.com/custom/update/query.html?filter=all&qt=winlogon.exe&nh=10&hitsceil=100&st=1&context=gbh

I've run full system scans with both Spyware Doctor and AVG's Free Virus Hunter, both with the most recent editions.

The only existing copy of Winlogon on my system is in my System32 folder...

Next suggestion?

(Thank you for the System32 bit, hadn't checked that)