Still running Windows XP?  Microsoft is officially discontinuing support on April 8th, 2014.  They will be continuing to release updates for Security Essentials until July 2015, but no more operating system patches to close the exploits that let the malware on your system in the first place.  http://thenextweb.com/microsof.....fromcat=media#!sntyt

If you don't need Microsoft Office specifically, it might be a good time to try switching to Linux and Libre Office.  I really like Lubuntu.  http://lubuntu.net/  It's lean and fast, even on old hardware.  And it's free!

What's that?  You need Windows to play the latest games?  SteamOS is free, and easy to install using Ye Olde SteamOSe http://directhex.github.io/steamos-installer/.  Valve is really pushing this platform and wants their entire library to be playable on it.  http://store.steampowered.com/livingroom/SteamOS/

But if you're determined to give Microsoft your money, Windows 7 is not a bad "upgrade" from XP, if it'll run on your hardware.