Media Center can't find digital TV channels for TV Wonder HD 600

I have a Power Spec Media Center Edition (XP 2005) computer with the MC Update Rollup 2 and SP3. I bought the ATI TV Wonder HD 600 PCI card to receive HD channels. Install seemed to go normally and the drivers appear in Device Manager as working properly. However, when Media Center tries to set up the HDTV channels (through an antenna), it first gets the Guide info, then scans the channels listed in the Guide, finds varying signal strengths from 1 to 4, but then doesn't stop, and instead goes back over the list and zeroes out all the signal strengths, and goes into an endless scan of the channels and never finds anything, even when I quit and restart Media Center or the computer. So I get no TV. By the way, I'm in Chicago and I'm using powered rabbit ears which work fine on our HDTV receiver.