Hello,
Besides those already mentioned, I also follow a few ground rules to ensure safe online ativities.
1) I use firefox to browse websites, and I configured it to balance security with compatibility (block popups, only allowing cookies from originating sites, etc.)
2) I only check email from my browser and only use a web based email like google or yahoo. I don't use email clients to read emails. Text based email clients are ok to use though. Google mail is particularly good because they now have antivirus protection as well as disabling images by default to prevent spaming sites from knowing that you actually viewed the pictures they send. AOL Mail is also good for this and they disable link by default too unless you specifically click on a link to un-disable the links and images). Plus AOL blocks spam goot.
3) Never download attachments or click on any links from your emails unless you are expecting the attachment or links.
4) Sometimes when I visit sites that I think are suspicious, I will use a web proxy for extra security (google for "web proxy" my favorite is Megaproxy)
5) Unless you live alone, always make sure your PC requires a login to use it. Make sure it locks the pc when screen saver is turn on (I set mine to turn on at 15 mins). Also make sure that in order to unlock or log in, you have to press ctr-alt-del.
6) If you suspect you have a keylogger on your machine...
http://dewasoft.com/privacy/kldetector.htm (google for more free or buy versions)
7) I don't personally do this.. but I know a few people that do. On their machine, they create a local user (non-administrator) account which they use to do daily activities like play games, email, surf etc. They will only log into their administrator account when they need to do... well, administration stuff.
8) If you have an always on connection, get a router with a built in firewall to deal with the riff raff of the outside cruel world.
9) Join a monastery and pray for your online safety. Oh, and while you're at it, have a go at world peace too.
If I think of more, I'll post. Until then, safe surfing!