Do you want to watch broadcast tv (antenna) or cable.
If its broadcast just make sure you buy a digital tuner this is what I have
its cheap $22 and I haven't had any problems with it the included software is crap just use windows media center its a great program.
O and if you don't have an antenna you will the need to buy one.
If you want to watch cable things get more complex an a lot more expensive you will need a cable card tuner the I have heard nothing but good things about the ceton infinitv 4
its around $200 i all so found this tuner from hauppauge
on Amazon its only about a $100 and has 4 stars on amazon but i haven't seen any reviews for it. the hauppauge has 2 tuners while the ceton has 4 which is over kill for what you want.