As always:
Keep your firewall up and running when browsing the net.
Keep Anti-virus software up to date
Don't open any suspcious attachments EVEN IF it comes from someone you know.
Don't click on any links in emails that tell you your credit card/password/blah is about to expire and you must fill out an online form.
Use popup blockers (free one at toolbar.google.com)
There are ways & means to tell if your machine has been compromised, I'll find the common ones and post a list here along with some common (& free!) fixes.
--
Gary Williams.