I have recently installed a media centre pc into my bathroom and I am willing to share the problems I ran into along the way and there solutions to help anyone who is deciding to under take this quest.
First of all make sure you buy a small tower or centre the smaller the better, I also planned were my centre was going, make sure you leave plenty of room for this as you will need a separate venting system leading out of the bathroom, you don't want to be cooling the system with steam from your bath.
I also installed an LCD TV for the centre, this is also used for watching DVD's; I have wired all this up to a separate fuse box to stop feed back and short circuits.
I have not attached a wire network cable but found wireless connection perfect for the job I have installed a blue tooth connection so my system is future proof you don't want to be ripping everything out once something new gets released.
I also found a spot under the sink cupboard to install two universal serial bus plugs, I placed my speakers on the bottom half of my walls because the speakers are exposed to less steam there.
I know it's not a step by step guide but at least it's a start.
The best advice I can offer is to be constantly aware of steam and water and fit your system around these rules.
Also find out if your system guarantee/Insurance covers water damage should things go wrong.