I vote new Rig because having an HP computer simulating device in your home is a sin. You are lucky the HP hasnt tried to murder you yet, I bet it has already murdered many of the games you tried to install on it. Save yourself and those you love and get that POS out of your house.
The 360 is a nice system, personally speaking I like it alot right now the games look amazing and I am happy to own one. But there arent alot of games for it right now and if I would have paid full price for it I wouldnt be a happy camper. Fight Night 3, Condemed and Ghost Recon (which comes out today or tommorow) currently are the only games I would consider buying if I was buying the system today. However PC has an amazing line up, there are so many different must haves on it that you cant go wrong, you have F.E.A.R, Call of Duty 2, Battlefield 2, Counter Strike Source, Age of Empires 3, Need for Speed Most wanted, Quake 4 (Which is much better than the Xbox one), X3, and many more even older games like GTA:San Andres which is sooooooooooooooooo much better on the PC (with the 360 controller) than it is on the Xbox or Ps2 versions.
Also if you build your own rig (Dont buy a HP, Dell or any crap like that please) than you can put your own parts in it and customize it to your liking, its really a fun and rewarding experiance in its own right. Plus you can do more with a PC than a 360, you can burn movies, surf the net, rip and make CD's, the possiblilies are endless, also you can upgrade unlike a console you can move up the ladder without buying a whole new console.