Swordsman, if your box is for personal use, the only reason to run as non-admin, in general, is to protect yourself from your own stupidity. Noninterleaved makes an excellent point: if you run a malicious program that tries to FUBAR your operating system, and you run it as administrator, it'll likely succeed. If you're logged in as a User, then the program runs with your credentials and won't be able to alter the kernel, device drivers, drives, etc.
Power Users are really best for business enterprises where you want techs to be able to do some tasks (such as adding a printer) but not have full control over, say, the company database server.
Power Users really don't have much place in the home world.
I run W2K as a member of the Users group. The last time I fat-fingered a command and nuked my OS, I convinced myself that always running as admin might not be a good idea.