I was playing FretsOnFire (Guitar Hero clone for PC) when Vista informed me that the program needed server access. I accidentally clicked block, and now I can't run the program, and I can't change the "block" into "don't block". Where can I change that setting?

what firewall do you have on the PC ??

run the software
look for a section about applications or programs and then you should be able to change its status

control panel/firewall/allow program then pick