Your current configuration: Windows XP SP3 with Microsoft Security Essentials + Sanxboxie and the free version of Emsisoft Anti-malware seems to be fine. I would personally feel that MSE + sandboxie is more than enough since MSE's protection is fine, but each to his own
I've used both MSE and Sandboxie in a LUA (XP) with great success.
IE 8 is a great browser (together with sandboxie) but the alternatives like Firefox and Opera are better. However, if you are happy with IE 8, stick with it.
I also used a program called "surun" which was like a mini UAC for Windows XP. Works great and does not bloat your system. You might want to try that.
http://translate.google.com/translate?u=http%3A%2F%2Fkay-bruns.de%2Fwp%2Fsoftware%2Fsurun%2F&langpair=de|en &hl=de&safe=active&ie=UTF-8&oe=UTF-8&prev=%2Flanguage_tools
You could also disable autorun and autoplay for starters manually or with a utility like tweak ui (scroll to the bottom of that page to download):
http://windows.microsoft.com/en-US/w...ads/windows-xp