Grats on getting up and running.  I've hardy ever been successful uninstalling and reinstalling different ATI drivers, most of the time I get the same application error if I do that.  Since these are mining machines, it doesn't take long for a reinstall (Win7 64-bit SP1 installed from USB stick, no updates or anything else).  I disable windows update and the rest of the action messages, install system drivers, a few utilities (VNC, Winzip, Afterburner, .net 4.0), then install the ATI drivers depending on card (6x gets 11.11, 7x gets 13.4, 7790 gets the custom one), copy my base cgminer and go.

The reinstall saves a lot of frustration.  A 450w PSU is enough to drive a single card, anything above that you may be running into power issues.