VNC not working on locked PC

Problem:
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I found that the VNC was not working on my PC(Windows XP) when it was locked using Ctrl+Alt+Del. Interestingly it did worked when the PC was not locked.

Solution:
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While installing VNC make sure that you select the option to install it as a service so it can be seen in services.msc

On installing VNC, it provides an option to install as a service. If it is not installed as an service, the VNC cannot be done on a locked PC.

Reason:
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This is what as told by one of my colleagues. Seems to be logical.

Windows uses API from gina.dll which provides the capabilities for locking/unlocking the workstation. API from gina.dll are accessible only by the windows services and not by other programs.