Doing this kind of work in addition to the 'day job' can be good for extra money. It has the advantage that you will probably visit the client in their own environment, at a time when they are not at work, so is more convenient for them. However, you do need to be upfront about your pricing, it can come as a shock to clients when you have spent several hours of what looks like just sitting there.
With virus removal, I find it can often take hours, not to remove the obvious problem, but to scan with 1, 2 maybe even 3 pieces of software to ensure it is clean. This is best done off-site, as you can do it whilst you are at home (you need to right business insurance policies) and then it doesn't matter if you have to leave it overnight.
A laptop I was asked to upgrade from Vista Home Premium to Vista Ultimate recently took 16 hours including getting it all sorted with Windows updates. It's a long story, but I only charged the client for 3 hours as it did most of the work whilst I was watching telly and doing other work.
You do need to price accordingly, remember your taxes, NI, insurance, transport costs (including uprated business use of your vehicle), tools, software. You'll work out you lose about 30% of anything you charge before you think about profit.