A question I have is, what types of protection does Microsoft Security Essentials provide? Does it scan and clean email, archives (zips, etc)? And, does it spot the viruses in files BEFORE you have to run it? I know with my current suite (Kaspersky), it's pretty good at finding and cleaning issues. However, I do find that it does seem to slow the system down somewhat, at least on boot up and log in.
What about monitoring web traffic? Kaspersky again can do this.
If MSE and some other "free" security software (such as firewalls, spyware, etc) work well, I might just have to make the switch.