Scriptno and similar products are good advice, indeed.
As for antivirus programs not finding trojans may be due to the long wake-up time of modern nasties. Some of them have been known to sleep half a day... Like my kids...
Several antivirus programs check things only once and then mark them as checked and if there's a sleeping piece of innocent looking code in the temp folder, saying something like "at shutdown download code from
this link and run before antivirus starts". The downloaded file may contain instructions similar to "at startup, disable <Brand> antivirus and run
this instead".
So, the antivirus may well be good and working, but it can be tricked. Removing temp files after surfing - or, as I do, automate the removal - associated with a script blocker prevents such delayed behaviour quite efficiently without crippling easy surfing too much.