MSE is not a good choice for Windows XP because it doesn't have some features like Network Inspection System.
Also MSE is not that good. Check this out => Kaspersky Internet Security 2014 Crowned the Best Anti-Virus in the World After New Tests. Yes, there is no antivirus that catches everything but however even MS confessed that MSE provides baseline" protection. I would replace it with Comodo Internet Security 5.12 (I don't like version 7), avast! 9 or Qihoo Internet Security 360 (to be sure that the system is protected from zero-day threats as well as other threats):
Also Qihoo and Comodo Firewall is a great combo:
The same is valid for avast! and Comodo:
Keep in mind that Qihoo had some privacy issues in the past but I would rather evaluate a product based on its effectiveness.
If you decide to stay with MSE then you can add MalwareDefender to controls what an application is allowed to do and not allowed to do. It monitors what each application tries to do, how it use the internet and give you the ability to block any suspicious activity occurring on your computer, although this would only be advisable for experienced users. More information about HIPS can be found here: What is Host Intrusion Prevention System (HIPS) and how does it work?
Keep in mind to choose carefully in order to avoid conflicts or instability caused by incompatible security programs.
Also having more than one "real-time" program can be a drain on your PC's efficiency...
If these kind of programs are difficult for you to use then you can use a standard user account. If you need administrative privileges to perform some tasks, then you can use Run As or log on as the administrator account for that specific task.