That's just computers for you
. Sometimes other apps you install conflict and just mess everything up.
Firstly then let's make sure your drivers are up to date - check your card makers website for the latest drivers and let us know if you need any help getting these installed.
This of course may not be the problem, but if we know you have the latest updated drivers then we can take a different approach.
Also you did not mention if you had any additional graphics software installed (such as the ATI Catalyst Manager) or if you had reset your displays settings.
If you require any help in the mean time just let us know.