Got a chea official OEM of win 7, reckon its not worth getting AVG or a 3rd party anti virus then?
Don't buy much for now, just get everything up to date and install Steam. Try to remove as much of the mostly useless fluff that manufacturers install on a new build. Things like trial software are a waste of space at best and can run in the background, slowing your system down or interfering with the things you actually want to run.
Peripherals (Xbox pad etc) are your priority because in a little while the Steam Xmas sale will start and your library will fill up very quickly. Get a few demos in the meantime to tide you over and give you something to test out graphics options on.
Virus/firewall duties are handled well enough by Microsoft Security Essentials. No need to install a load of other crap and it's free if you have a "genuine" copy of windows.