the problem with the 'blast it with a reformat and put on a clean copy' brigade is that it does not work for some of the more sophisticated virii which lodge on the hard drive in places that are not touched.
Sorry BOAC but someone has been telling you porkies.
(1) It is MUCH safer to reformat and re-install than to try to eradicate a deeply embedded virus, high risk (e.g. rootkit) virus, or a system with multiple viruses eating away at it. It is the only way to re-establish trust with your system, and in these days of online banking etc. it is much wiser to be safe than sorry.
Anyone who tells you otherwise doesn't have a clue what they are talking about, and I hope you were not paying for their services.
(2) If you re-format correctly prior to re-installing, your statement about "virii which lodge in places that are not touched" can easily be shown to be utter nonsense. If you are really worried, just replace the physical hard drive ... they are so cheap these days....