Be sure you have the latest ATI drivers r setup s/w.
While the setting can be correct for your original dual or ext. monitor setting these are not always "auto setting" when yet a different monitor is used. Basically, you've confused the system and can only fall back on the original setting. Also, having a 4yr. old laptop may compound the problem as when it was released the ATI video just became less capable as time marched on even with newer s/w patches/drivers.
This is weird: after I use my TV or overhead video projector as an external monitor (successfully), my regular external monitor (LG L194WS) settings go bad. Only the mid part of the picture shows, while some 15% of both sides are black, covering parts of the picture, which is also a bit narrower than before. No amount of setting the ATI video card or Windows will help. Yesterday I had to go to a previous Restore point to fix this.
My laptop is a 4 years old Presario V2000, WXP SP3, 1G RAM, Turion 64, always up-to-date and clean.