1. About uninstalling other antivirus programs before installing MS Security Essentials, I read this on one of their forums. Not sure what "at your own discretion" means.
Important: "Having more than one Antivirus installed at the same time can cause stability and performance problems, please uninstall all Third-Party Antivirus applications at your own discretion before installing Microsoft Security Essentials."
2. The different sizes of download modules has to do with which OS one is running, I believe. The 8 MB one is for WinXP which is what I want.