That is a driver issue and an ATI one. There have been some reports of incompatibility between HD5xxx cards and some older motherboards with PCIe 1.0. Some mobo manufacturers have solved that with a BIOS update. Just wondering whether you are experiencing a version of that.
I had a similar problem with my system. Windows 7 64 professional and catalyst 1.3. I think it was caused by an update to adobe acrobat. No idea why. I uninstalled acrobat completely, and the problem went away
I installed the adobe update late on the 17th, and when I woke up on the 18th my computer wouldn't.