Take a look at MediaPortal. It's open source and I use it as my HTPC front end. I haven't used it's TV EPG because I have BeyondTV, but you might want to give it a try.
Another free application worth taking a look at is GBPVR. I haven't any experience with it, but have heard good things.
Lsat I know, the EPG support (zap2it) on GBPVR is long gone..