http://www.free-av.com/
Antivir - Among freeware, I think it has good detection rates,
lightweight, but a download side is that there is no incremental
updates, worse every 2-3 weeks or so you need to download the
whole program again. Killer if you are using dialup etc.
I also find that even if you are on broadband downloads are very
slow , servers are often down etc. AVG used to be bad in this
area, but Antivir is even worse.
http://www.avast.com/
AVAST - Good detection rates, excellent features including
"webshield"(though buggy), networkshield, plugins for instant
messangers,P2P etc Unlike Antivir,AVG, downloads are small and
fast.
The catch? It is somewhat heavy in terms of memory use compared
to Antivir and AVG, though I personally find it acceptable.
Together with AVG these are the big 3, with resident scanners
and non- time limited deals.
There are(were?), one year 'trial's of other AVs out there, but
I don't think you are looking for those. On demand scanners are
another option.