ever if u've antivirus/anti spyware/anti adware/firewall
it still possible to get your PC infected,, IE / outlook express and few other common internet application would be vulnerable to to certain bug that would plant and execute malicious software (malware) on your machine just by surfing a specially crafted site....
normally .. regularly updating your software would be enough..but then weeks later..someone would disclose another bug on IE (they just hate microsoft..dont ask me why) .. consider switching to other web browser..heh