Any virus checker is only as good as the last update. This is where most people fail and then have problems.
For XP-based systems I used to recommend McAfee but it has become too 'greedy' and the company seems to think that it owns your computer. It is an OK system, but for the money it should be better than it is. Norton is just a pile and should be left in the bin. If you want free, then AVG from Grisoft is the one to go with. If you are willing to pay (which I would recommend) then you have a choice of F-Secure, Kaperski and Windows One Care - all of which do the job well.
For Vista-based systems, the only one we have not had problems with is Windows One Care - all the others have given no end of grief.
For PocketPCs (yes, the pond-life got these too) we use F-Secure.
Hope this helps