We've just got an iMac in the office. Can I just say that I have no idea about Macs at all! 
We've got it connected (bound) to the Windows Domain without problems but now obviously need to login to that domain account but for some reason we can't.
Does anyone have any ideas or tips I can try as I'm at a total loss.

We've got it connected (bound) to the Windows Domain without problems but now obviously need to login to that domain account but for some reason we can't.
Does anyone have any ideas or tips I can try as I'm at a total loss.