PCs Vulnerable to Spyware, Viruses
Unlike with previously revealed vulnerabilities, computers can be infected simply by visiting one of the Web sites or viewing an infected image in an e-mail through the preview pane in older versions of Microsoft Outlook, even if users did not click on anything or open any files. Operating system versions ranging from the current Windows XP to Windows 98 are affected.
My computer was shutting down from spam/ad/spyware attacks regularly until I switched to Mozilla Firefox two months ago. Now, I never have any problems. It's a free download and will save you a Bill-Gates-sized headache.