I have long thought about doing this too, it's been one of my dreams for a long time, although there is a fair bit of competition going around. I also work for a repair company which I took as a part time job whilst I study at college to get some industry knowledge on how things work at the business side.
The best advice I can give is to price your work accordingly. Find out what other local companies charge for virus removal, hardware testing and installation etc etc..
Also in the process of reading this book. Starting a Computer Business - Building a Successful PC Repair and Service Business
It is aimed more at the American market but still contains relevant information to computer repair startups if you live elsewhere.
As for advertising, a simple website is enough to advertise details such as services provided, hours, cost and contact details. What about local newspapers too? You can get a few hundred business cards printed up for a few quid to hand out to customers.