AFAIK there is no need for antivirus in Symbian fones, unless u install each and everything u recieve from blutooth, just keep BT off and do not install, ANYTHING you did not ask for in the first place.
6600 has very low RAM, an AV will make it way slower.
If you are not convinced - A plea for sanity - *3lib.ukonline.co.uk/viruses.htm