ESET Smart Security
of course - reliable, consistent protection for years, and you don't even notice it is running!
v5 looks even better, supposed to be even lighter and better interface - not upgraded to it yet (all users regardless of it they purchased v3 or v4 will be eligible), waiting for it to come out dev testing!
As you may notice, money can b an issue with ESET, as it £20 for PC's, pennies in all honesty. PC World are currently charging £49.99 for it (though they do include free ESET Mobile Security with theirs - which is only handy if you have a Windows Mobile phone lol).
I see you gave up on F-secure in the end little_pob, quite interesting, I was running it last year, and apart from a clunky interface, I didn't notice any problems with it. They must have changed it, for the worse.
Norton - stay away, absolute shite, Kaspersky is alright, but can cause very noticeable slowdowns when it is running updates, the other problem is on some PC's it bogs the computer down as if the PC had a virus. The missus was running it before I go the new ESET license, and it was terribly slow,. thought she had malware, removed Kaspersky and all was back to normal, scanned with Mbytes etc, and no problems found, so can only assume it was Kaspersky. She is now running ESET without issue.
Btw, 7dayshop has changed the price to £15.99 delivered for ESET, that is quite simply amazing, a fiver a system!!