I've never seen a pc logo for any PC game is their even one?
I think this is the symbol most likely in use at the moment, it seems to change
http://www.google.com/imgres?um=1&hl=en&sa=N&rlz=1C1AVSH_enUS459US459&authuser=0&biw=994&bih=608&tbm=isch&tbnid=H2YpPrgPeYD21M:&imgrefurl=http://awinash.0fees.net/personal/&docid=c3RBjWKdSIoZ7M&imgurl=http://awinash.0fees.net/personal/images/stories/games-for-windows-logo1.jpg&w=400&h=400&ei=LIuDUKK8KOq80QGrsoDgDg&zoom=1&iact=rc&dur=485&sig=102256819200540021074&page=1&tbnh=163&tbnw=181&start=0&ndsp=14&ved=1t:429,r:1,s:0,i:75&tx=75&ty=95
As for the ones that used to be used that'd be the next link
http://www.google.com/imgres?um=1&hl=en&sa=N&rlz=1C1AVSH_enUS459US459&authuser=0&biw=994&bih=608&tbm=isch&tbnid=M8oH1Mp_Rwy4bM:&imgrefurl=http://64.239.241.166/ko-KR/Gamebryo/Platforms/PC/&docid=Vw-HWZm4Sg4C4M&imgurl=http://64.239.241.166/Global/Images/Platforms/3/pc_logo.jpg&w=190&h=145&ei=LIuDUKK8KOq80QGrsoDgDg&zoom=1&iact=rc&dur=564&sig=102256819200540021074&page=1&tbnh=116&tbnw=152&start=0&ndsp=14&ved=1t:429,r:3,s:0,i:81&tx=105&ty=55