Just to be sure
Just to be sure, you are aware that there are plenty of known threats to XP which require absolutely no user interaction at all, beyond being connected to the internet? Further that there are automated exploit programs regularly employed by people to run continual scans of computers on the Internet looking for these vulnerabilities.
I also don't see how a computer is a cheap alternative to a radio. Even if you're running headless (no monitor), you're looking at probably somewhere in the neighborhood of 100-150W of power used. Compare that to the average clock radio which is probably 10-15W or less. If you have a somewhat larger stereo you might get up to around 50W give or take, and a full on surround sound system would probably rival that of a computer.
Thus the computer is a much more expensive proposition, even in a pretty best case scenario. You may not have any up-front costs associated with buying a stereo of some sort, but over the long term you will spend much more in the way of added expense on your utility bill.