Based on the general difficultness of locating the free version of AVG (not actually mentioned on www.avg.com - only on the secret http://free.avg.com), along with the nags for upgrading to the professional edition – and finally following the pcworld review comparing AVG Free to MS Security Essentials, I’ve made the switch!
It seems MS Security Essentials was even coming out on top in the comparison.
It seems more lightweight and fast too, and it's free – nice. If you wish to download it you can get it from here.