I'd recommend Avtivir as I prefer it to Avast. It has slightly better virus detection rates (in recent tests) but either way they're both much better at picking up viruses than AVG.
There shouldn't be a reason why simply opening a spam email such as the one you did should give you a virus, not unless you're using a fairly old and unpatched email program. What email program do you use?
I personally wouldn't bother with OneCare. It didn't do very well in the latest virus tests I mentioned. It was one of the worst performers. It's very unlikely running it before running Antivir or AVG will achieve anything.
"Should I Clear all the System Restore Points that were after the one I used ?" I would, once you're sure you're infection free, as the trouble with System Restore is that it can backup and restore a virus.
There's a link to tp a pdf of the latest virus tests on this page if you wish to compare antivirus programs.
Link Of the ones discussed here the results were Ativir (Avira) 99.4%, Avast 98%, AVG 94%, and OneCare (Microsoft) 90%.