I've just got this working and it's amazing! I was going to spank £80 on a new HP printer to get AirPrint but found this post for free.
I have a wireless HP, PhotoSmart C4780, and a Windows Vista PC.
I simply downloaded the program, ran it (must run as administrator, grrrrrr wasted 10 mins on that), share the printer and add the AirPrint.exe to the firewall.
The printer then just pops up on the AirPrint list. I guess the only downside is that ur Pc has to be on.