There are several such questions, which cannot be answered at the moment.So create an image of your Windows XP and maybe even back up the Activation Token files.So it will be imperative that you install a good firewall too, apart from an antivirus software.
Given the ground realities where a bulk still use Windows XP, and considering the fact that there are many still many home users from various parts of the world, who just would not be able to afford to upgrade to a newer version of the Windows operating system, securing their computers is going to a big challenge!
Even today, Windows XP holds around 20% of the operating system market share. In Africa, Windows XP still commands a market share of around 33%. In India it is around 30%, whereas in China it is 51%!
A Software Update Checker like Secunia Personal Inspector will scan your computer for installed software.
You will thus be protected from vulnerabilities in these software. Since Internet Explorer too will not get updates, it might be time to drop IE and move to an alternative browser like Firefox, Chrome or Opera. Email is a common vector used by malware to get on your computer. You can surely download attachments you are expecting from friends, relatives & associates, but be very careful of the mail forwards which you may receive even from your friends.
And once Microsoft stops providing security patches and updates to Windows XP SP3 and Microsoft Security Essentials, it will become an open playground for hackers and malware pushers.