1) There are loads of bike sheds. The ones by the SU and the Maths department are sheltered, but the ones near the science and humanities blocks are not (though there may be other sheltered ones in the area, I'm not sure). These are just the ones I know of, and I am sure there are lots of others.
2) You should never by antivirus software, you can get all the antivirus you need from the free utilities the companies offer. I have heard good things about the AVG one and Microsoft Security Essentials.
3) You get a practise room by turning up and hoping that there is a room free - there is no booking system. During the day it is not uncommon to see queues of people trying to get into one but I have always found that if you are prepared to hang around until 7 or 8 pm, or come back onto campus, there are usually one or two free that you can just walk into, or if there aren't you will just need to wait a few minutes. All the pianos are real but are of varying degrees of quality - if you spend a lot of time in the practise rooms you will quite quickly learn which ones to try to get into. All of them are servicable enough to practise on if it comes to it. Finally, there is an extra music building on the Westwood campus that is maybe a 20 minute walk from the ones in the arts center but in all the times I have been I have never actually failed to get a piano. However, the pianos here are generally in a much worse state and you might not like to practise on them. If you have some free time it is worth going down though, because there are a couple of decent ones including quite a nice grand piano that is fairly often free.