It's actually not very difficult to setup a web server. For Windows, there are bundles you can install which include Apache, PHP, MySQL and a few other goodies. I've tried it at home with no prior experience and found it relatively easy to setup.
However, I too, would advise against it. As mentioned, bandwith may be a problem. Reliability too. But for me, the most important issue is security. There's just too many holes to plug and I, myself, probably know less than 1% of them.
If I had an extra PC, I'd install Linux and practice how to use it. I'd also use it as a music server since I own a Squeezebox2
and stream music wirelessly within my home. Or maybe it would be a file server, in general.
Anyway, I'm sure there are lots of neat things one can do with an extra PC.