How will crunching affect me ?
Your IQ will increase by 150%, the opposite sex will fall at your feet and you will win the lottery

Thats what I've been told anyway
Seriously though, it shouldn't have an effect on your system as it only uses spare CPU cycles.
I surf and download at the same time . Or I watch .avi files in power DVD .
Loads of us here to tons at once and it doesn't slow anything down, if your system is busy the crunching software just won't kick in
Is the crunching software easy to set up .
Yup, really easy - there are 2 choices to use but we can guide you through either if needs be
How will it affect what I do .
It shouldn't have any effect on your system, the default WCG client limits the max CPU percentage that the client uses at something like 60%. For example, say you are browsing the internet and using 20% of your CPU, WCG will use an additional 40% to crunch. The limit at 60% means that you won't risk heating up your CPU (although that isn't really an issue with well built PCs).
Will it slow down me surfing .
Nope
Will it affect me movie watching.
Nope
Will it use up lots more electricity .
It will only really use more if you leave your PC on for an extra period of time.
Can I just do it every now and again .
Of course you can

If you don't fancy having it running all the time you can install the default WCG client and make it only run as a screen saver. There's no-one pressuring to run it all the time, just whenever and however you want
Will I get a free crunchers T-shirt (this is a very important Q )
If I ever get crunchers T-Shirts done then yes

I did look into it a while back (either that or mouse mats), but they came out quite expensive. If a company decides to sponsor the team or something, then yes