Choose your software wisely
If you use Microsoft's Internet Explorer, it's time to let go. Internet Explorer 8, the most recent version available for Windows XP, is already several generations old and will no longer receive security patches. Google Chrome will continue supporting Windows XP until at least April 2015, while Mozilla Firefox has no announced plans to stop supporting Windows XP. So switch to Chrome or Firefox and you'll have a secure, modern browser.
Most antivirus solutions will still continue supporting Windows XP. Even Microsoft's own Microsoft Security Essentials will support Windows XP until July 14, 2015. Antivirus-testing company AV-TEST asked 30 different antivirus companies about their plans for Windows XP support and all of them committed to support Windows XP until at least April 8, 2015. Most committed to supporting it for even longer, into at least 2016.
Be sure you're using an antivirus program that's actually receiving updates, though, because that expired copy of Norton isn't going to help you. An antivirus app isn't a foolproof solution, and Microsoft warns, "Our research shows that the effectiveness of anti-malware solutions on out-of-support operating systems is limited." Still, having some type of third-party protection certainly won't hurt.
If you're still using the now-defunct Outlook Express, you should stop using it right now. If you really love the Outlook experience, switch to the full version of Outlook included in Microsoft Office. Mozilla is still supporting Mozilla Thunderbird with security patches, though it's unclear how long they'll support Thunderbird on older operating system. Of course, you can always just use a web-based email service in Chrome or Firefox.
Microsoft will also stop supporting Office 2003 on April 8, 2014. If you're still using Office 2003 -- or, even worse, Office XP -- you should update to a newer, supported version of Office for improved security. Yes, this means only ribbon-ified versions of Office will be supported. Sorry.
Remove insecure software
The Java browser plug-in is extremely exploit-prone on any operating system. Unless you really need Java for a specific purpose, you should uninstall it. If you do need it, be sure to disable the browser plug-in and keep it up-to-date.
Other browser plug-ins are also frequently targeted by attackers. Adobe Flash and Adobe Reader are particularly crucial, so keep them up-to-date. Modern versions update themselves automatically, but older versions didn't even check for updates. If you don't need these applications, you should probably uninstall them to lock down your XP system as much as possible.
PCWorld senior writer Brad Chacos got sick of the constant security klaxons and tried living without Java, Reader, and Flash, discovering that it should be surprisingly easy for many people.