I'm impressed with Microsoft Security Essentials. I assume the developers know a bit more about the inner workings of windows beyond the API the rest of the world is given. MSE is very fast, it doesn't bog things down the way other anti-malware stacks invariably do.
I also agree with tweaking the search indexing. I usually shut it off entirely, but on occasion I'll set it to rifle through a very specific set of data, if the user has a lot of stuff and does a lot of searching through it.
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