Monitor not recognised

Hi,

I've got a problem with my desktop not recognising the make of my monitor. I am using an Acer S240HL monitor, but Windows 7 shows it as "default monitor". I've booted into my backup clone, and there the monitor is correctly identified. I've tried reinstalling the monitor according to the manufacturer's instructions with no joy. I tried installing the monitor on another base unit and it was correctly identified.

Any suggestions?