I hate having to wait for my PC to boot up as well, which is why I leave it on 24x7 and have it folding away. If I don't have it folding, I just leave it on standby, where everything shuts down with just enough electricity being used to keep the contents in memory. I also have the video and hard drive set to power down after 20-30 min. Uses minimal power and the PC wakes up and is usable again in 10 sec.
Not sure if I'm correct, but I think your LAN card will be asleep too, so there are no Internet connections active when your PC is asleep. Even if that's not true, if you have a firewall and have your file sharing turned off, it should not be a problem.
Setting your PC to hibernate is also an option, with the contents of your RAM being written out to disk before hibernation. It takes longer to wake up (still less than a minute) and uses even less power than standby.