There are a few ways of getting it. (Sky TV online, online tv service...) But I think the easest (sorry my spelling is rubbish) way to get it is by a USB dongle. You will get all the freeview channels plus the interactive stuff. Plus if you have a decent sized hard drive you can record it to your pc. The only downside (as stuartli said) is that you need a decent signal. You can also get TV cards for your pc and these are helpful because on some of them you can plug in your rooftop ariel and use that.
>>I am still at a loss to understand why anyone would want to watch TV on their PC.>>
Many reasons. One is that you can listen to radio or watch Sky News etc with a smaller display fitting into the top right hand corner of the screen whilst using the system; secondly, it prevents conflict with the other half about what to watch on the main back room TV and, thirdly, I can watch both Champion League matches on ITV on Tuesday nights when they are on...:-)
There are several others, but minor ones in the general mix.