App running on Vista not working on Win 7

I have a custom app which is written in C#, and its basic job is to talk to USB port,
send/receive data from an external device.

This works fine with Vista but not with win7. Application refuses to launch/crashes soon after.

Are there any API changes to Win7 which would cause this kind of error?

I am not a expert in windows programming, just starting up with this assignment to port this app from vista to win7.

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