The first thing to do is remove the filter for a short while and see what happens. You will probably find that it needs replacing anyway.
You generally get this type of smell from bacteria that builds up in the system when you use the A/C. If you go to Halfords, you can get an aerosol treatment to cure this. £11.99
http://www.halfords.com/webapp/wcs/stor" onclick="window.open(this.href);return false; ... yId_165632
Two things to know about A/C:
- It should be run for at least 30 minutes each month, to prevent the seals drying out.
- Turn off the A/C a little while before you finish your drive, this allows the system to dry out and prevent the bacteria forming.