This is a great product — Audio Hijack.
Allows you to grab audio from nearly any source, as long as it is playing on your computer. I’ve used it to capture some music performances from the Henry Rollins Show website. You can then use other audio editing software like GarageBand to put it in the format you’d prefer. Really cool.
Only for the Mac — so get yourself a Mac.
Speaking of music from the Henry Rollins show, if you like Thom Yorke, of Radiohead, check this out.
In the Windows World, Total Recorder does the same thing for XP and before.. I used it for many years for just the type of recording you describe and was quite pleased with the results, but at this time it does work as well with Vista.
For Vista there are a number of products one can buy that should work but I was too cheap, so I used the shareware Hi-Q ($0.00).