Every program contains errors. Some of them are relevant to security and become known at some point. And as soon as a security problem is discovered, malware authors try to exploit it. This makes bugs in email programs, the operating system and programs used to open attached files, especially dangerous. For example, companies were attacked recently with emails that carried handcrafted Word files. Because of a security hole in Word, they installed keyloggers and backdoors on the computers on which they were opened.
Windows patches are available at www.windowsupdate.com. However, it would be better to activate the automatic update function in Windows XP (Service Pack 2) so that updates concerning security are automatically downloaded and installed. Outlook Express, which is part of Windows, is automatically updated along with the operating system. In the meantime, updates for Office programs like Outlook and Word are also distributed via the automatic update service. The developer team at www.mozilla.org regularly releases new versions of its browser suite and the email program called Thunderbird.