I've not heard of the computer virus calls before but if you receive legitimate, albeit annoying, telemarketing calls just tell them "sorry, this is a business number". They are not allowed to target business numbers to offer personal / domestic services and are usually very quick to remove your details from their lists, which would otherwise be sold on. The potential fines for them are astronomical.
Remember that the majority of cold callers hate their jobs and the constant abuse they receive, and so if you are polite and treat them like human beings (difficult I know) they are more likely to oblige.
On most mobiles you can block some numbers, any junk calls I get where their number comes up, I just add to the blocked caller list.
Home phone is a pain in the backside though, a couple of people (me Grandad for one) that ring us use a witheld number for some bizarre reason.
You have signed up for the 'Telephone Preferance Service' etc ?
I have a friend who lives for these calls, they are the highlight of his day. I think his record for keeping a cold caller or scammer on the line is approaching an hour!
My favourite was when the scammer asked him to open up windows he said OK and put the phone down for 5 minutes, he then asked the scammer if he could close windows as the snow was coming in!