Though not open source, Opera has to be the fastest browser in terms of rendering that I have come across. I do have to fire up Netscape for many things such as Java, but then again I am only using v5.0 of Opera.
There are a number of anti-virus apps out there. However, most are for checking email attachments. I did see one that appeared to be stand alone. I believe it was called "AnitVir". Since I have not used any of these apps I am not able to provide any information other than they exist. Take a run over to freshmeat.net and search for "virus".