Can you get infected (virus, spyware, malware) from simply visiting an Internet site or do you have to actually download something. I know you can get infected by opening up emails.
How accurate or helpful are the internet site advisers such as McAfee or avast?
Any help is appreciated!
Short answer, Yes you can.
Sites are hacked so that the act of simply visiting a site could run a script that loads and downloads remote content (driveby download).
The avast web shield is blisteringly hot on detecting hacked sites and depending on browser the script shield would also be scanning scripts, so you are pretty well protected by the web shield. The Network Shield is also on the look out for attempts to connect to known malicious sites, a list of these is contained and maintained in the virus definition updates, again another level of protection in avast.
So personally site advisers are pretty weak in comparison to avast, in fact the McAfee site adviser is very poor as its database can be very old.
You could also use firefox with the NoScript add-on, which prevents scripts from being run unless you give permission for that site. Driveby downloads rely on scripts to do that and if A) the web and script shields scan and B) NoScript blocks scripts by default, then you are pretty well protected.