Windows Updates: regular updates are crucial.
Go to http://windowsupdate.microsoft.com/

Install Antivirus Software: Find a good one and update weekly.
Try http://www.grisoft.com/us/us_index.php

Use a Firewall: Your PC may not survive without one.
Try http://www.zonelabs.com/store/content/company/zap_za_grid.jsp

Ignore Pop-ups & Ads: Unless you trust them 100% ignore them.
Try http://www.panicware.com/

Block Pop-ups: Get a good pop up blocker – usually bundled with good AV software. Try http://www.panicware.com/

Increase your Internet Browser’s Security settings:
For help & advice go to http://support.microsoft.com/

Don’t Use Debit Cards: credit cards may be safer.
See http://www.webdevelopersjournal.com/articles/card_fraud.html

Beware of Spy ware and Ad ware: check out http://www.pcworld.com/downloads/file_description/0,fid,22262,00.asp