There are a few ways I could see this going. A 175 Watt PSU is pretty small considering the demands of most computers and its possible that the system can't boot fully because its simply not recieving enough power. This would seem to be re-enforced by the fact that the system does boot up for you when the card is removed. It doesn't however explain the fact that you were able to boot the system once with the card installed. If you have CD Drives in the machine you could try removing power to both of them and booting the system to see if that allows things to function, and that would also suggest very strongly that it is indeed a power supply issue. You could also try removing any PCI expansion cards and leave just the harddrive, the memory, the processor, and the video card with power to them to see if that load can be handled.
Something else to try might be booting the system into safe mode and uninstalling the video card drivers and then booting the machine normally and seeing if it will get into Windows that way. Then once you're in Windows check out ATI's website for the most up to date drivers available and see if that takes care of the issue for you, otherwise I'm inclined to believe your PSU just can't handle the load.
