Let me turn the question around. Since you have XP pro you can run an non administrative account with a full software restriction policy. You could have no other security, you could be deluged with all kinds of malware, but since it can't install, it could not infect.
Unlike Vista which is a more secure OS, XP is full of malware vulnerabilities. Regardless of what security you run, much of it is signature based, and with new malware entering the wild on a daily basis, it takes time to prepare new signatures and more time to get those signature updates into your computer. Leaving any security set up vulnerable to these so called zero day attacks.
Running a non administrative account in XP simply makes sense because it greatly reduces your risk while leaving your existing security set up in tact. And with fast user switching, you can switch to an administrative account easily when you need to.
You paid the extra bucks to get XP pro rather than home, you can run that full software restriction policy, and so why not take advantage of it? Evading malware is always a probability game, there is no such thing as perfect safety, its just a matter of reducing your risk. And using a non administrative account is probably the biggest single step you can take in reducing that risk with win XP. Mech Bgon has instruction on how to set up a full software restriction policy in the security resource thread. It only takes ten minutes or so.
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