If you live in the West London area I highly recommend Geek Squad.
My computer had nasty spyware, viruses etc and it wouldn't stay on, kept error messaging and closing.
I called Geek Squad in the afternoon, booked an appointment for the next morning at 10am, they promised that this was an actual time and not an estimate and at 0955 they arrived.
They went through everything they did, explained everything, fixed it all, and I was very happy.
They are great, good knowledge and very friendly. The benefit of them coming to you is also useful as it means you don't have to send or lug your computer anywhere.
They are new in the UK but have been going in the US for ages, reasonable in price too. I think a complete overhaul of my computer and external hard drive including everything was about £250. The price is to do the job and not per hour too, my guy was here from 10am till 2pm and so I think for IT techies that is very reasonable