Connect to PC via USB

Hi all - hope you can help.

I'm trying to connect my Nexus to a Windows 7 PC via USB, which has worked before but has now stopped working.

When I plug in the USB cable I ge the USB Mass Storage screen (green), click the 'Turn on USB Storage', its busy for a moment, flashes up the 'USB Storage in use' message (orange) and then reverts back to the first page (green).

Now I am assuming that the problem is that something else is using the USB and thus it can't be unmounted (I think), how do I detect what is using the USB storage?

Phone settings (about) say Android 2.3.4, Baseband I9020XXKD1, kernel 2.6.35.7, build GRJ22.

Cheers,

Coalman