A friend who fixes these informed me that some of the freeware applications are just as good as those you pay for. But they all have their "achilles heels". Decided the answer (or part answer) is to change occasionally, or have several independant scanners of various types, especially anti-spyware.
A2 (squared) free is a good (but slowish) stand-alone malware scanner that can be used in addition to your monitoring antivirus, I'm using Avast for that, and find it pretty good. Also using Ewido antispyware,MS (giant) antispy (not Windows defender), AdAware and Spybot. None of these appear to cause conflicts, none are too demanding to use or configure, some require a bit of attention, ie manual updates/scans, which I've found a good habit to get into. It's keeping me aware...a bit like trying to stay aware of human health nasties that keep mutating.
Norton tried to cripple my new computer, and wasn't exactly a clean un-install. Even after a re-install of the OS, there are still traces.
PS (edit) Trend Micro also have an online free virus scan, very good if concerned that something might have slipped through.