I mostly agree with chipdeath, although if you do have a good HDTV - say with 1080i - in my opinion it's not all that bad to use as a monitor for simple stuff like surfing. Games can be a riot, too.
But don't expect to edit images or do serious work on the bugger.
And if it's not an HDTV, you can pretty much only use it for console ports with big text, like Need for Speed titles or what have you. It will be very, very blurry, and you probably won't be able to read simple windows text even at 640x480 res.
this is entirely possible and actually quite easy. as others have said tho, you will want a front end (like MythTV) that will make things like reading text, and accessing HTPC style apps easy, and in general make the system functional.