How to find the installer already running or not

Hi all,

I have one NSIS installer. If a user once executes the my installer, then on further execution of the same installer while the first installer is in progress, should prompts a message like "Installer already running" or should not execute anymore till the first execution ends.

So using NSIS script how can I trace that my installer is already in execution state or not?

Regards
VCR