"I don't know how I got a virus, I have a virus checker installed". Upon inspection that is true, the customer did have the software installed but with a few problems which rendered it useless.
The common issues I find is that people are still using the trial software that came with the machine but have left the subscription to run out. Most anti virus software would not work once this happens, after all these companies will not give you anything for nothing.
If you do want to renew the subscription of the software because you like it then don't just click the renew subscription button. You would get a better deal shopping around or just remove the software and try free anti virus software.
There is also a selection of people who have anti virus software installed but don't update it. They take the time to download for example AVG 2009 then leave it the same.
Once all these have been sorted and you have an anti virus program that works, free or paid then set the machine to perform a scan at 2am in the morning. After all everyone's machines are on at the time aren't they....................oh hang on.
It would be advisable to set the scan when the machine is on so it can at least go through the scan. Most software allows you to either set it on certain days in the week or just weekly. If you set it every day then it may be on during the time allocated.