XP Pro.'s winlogon.exe not locking my system?

Hello.

I have three different custom built machines with old, updated Windows
XP Pro. SP(2 and 3) that have ATI Radeon video cards (9600 AIW, 9800 Pro
AIW, and HD 4870) and ATI Catalyst drivers (latest to the old legacy
versions).

I noticed sometimes they won't/didn't security lock correctly from their
defined screen saver's times. Even press winkey+L does not lock. I did
see winlogon.exe running in XP's Task Manager. The only way to fix this
problem is to reboot the computer.

Is this a known issue and is there a way to fix it without rebooting
like restarting winlogon.exe process? It seems like this is related to
ATI video cards and its drivers in XP Pro.? And yes, I scanned for
malwares with multiple updated softwares. They all showed my systers as
clean. Event logs don't show anything about winlogon.exe.

Thank you in advance.
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