Password rejected using LockWorkStation

I have a shortcut on my taskbar to lock my workstation while I am away from
my PC:
%windir%\System32\rundll32.exe user32.dll,LockWorkStation

This has always worked fine before. When I click on it, I get the typical
logon screen that I usually see when I boot-up Windows XP pro.
Lately though, the screen pops-up like always, but my password doesn't work.
I am forced to re-boot the PC, and the same log-on screen appears, and now
the same password does work.

Any thoughts on this?

Thanks,
Steve