Originally Posted by Highly-Annoyed
OK, I hate to be the only cynic here, but I think you already found your problem and stated it in your first post. You let your friend use you rig for several hours... He probably either did something to the computer deliberately or accidentally that has caused it to stop working properly. Honestly, from what you've said, your computer was working perfectly, then you let your "friend" use it for several hours, now its operating far from perfectly... It's amazing how "friends" can be so careless with stuff that isn't theirs... I personally never let anybody use my rig for more than 10-20 minutes and never leave anybody using it unsupervised for more than 5 minutes... Maybe people might think that's a bit odd, or paranoid, or distrustful, but I can honestly say taking that attitude has saved me a lot of time, money and headaches...
Your problem sounds to me like your graphics card has sustained some damage, most probably through overheating/overclocking/stressing it too far. It is possible that it may simply have been faulty at the time of manufacturer and somehow got passed quality control, but I would guess that the current problem occurred whilst your "friend" had your PC, although I doubt you'll ever prove that.
RMA, if possible, would seem like your best (and quite possibly, only) option.
I have to say im with you there on some of the points you made but we cant JUST blame his mate lol..it could seriously be that the game he played stressed the already overheating card and by chanced killed it so dont just say to your mate '' OI YOU LOOK YOU BLEW UP MY G-CARD''.