^Depends on what kind you want.
http://ncix.com/products/index.php?minorcatid=1066
Also what kind of components are inside your computer? You'll need that to know what minimum wattage you need. If you're not sure I'd say go for anything around 450-600W.
To be honest though, if you can get into windows and actually just get a glimpse of the desktop, it may not be your power supply. It's always a safe bet to change that first. Do you see anything before it boots like the POST messages and stuff?