winlogon.exe consuming 50% of CPU time

Running WinXP Pro SP3.
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I did some checking yesterday to see why my PC was "slow" and found that
this process was using !50% of the CPU time. Did a reboot, same thing.
Googles it and saw I might have malware so I ran Symantec AV, Windows
Defender in full scan, Sysinternal's Rootkit Revealer, and Windows Malicious
Software Removal. They found nothing.

I ran Sysinternal's Process Explorer and found the following:

winlogon.exe >> Properties >> Threads
TID 3108 consumes ~52% of CPU time and CSwitch Delta is ~160, and Start
Address is winlogon.exe+0x39156, and Context Switches is ~68,000.

The total thread count for this process is 22.

I've gone through msconfig to pare down what auto-starts with the same
results.

What else should I check?

--
Mike Webb
Platte River Whooping Crane Maintenance Trust, Inc.
a conservation non-profit (501 (c)(3)) organization
Wood River, NE