MBSA uses Microsoft Update as it can scan for Office security updates along with IE8, .Net, etc - pretty much all MS software. Difference is, MBSA only shows security updates, not the optional and WGA updates that might be specific to your computer/hardware. For example, if I use Windows Update in Win 7, it will show me update for my printer driver and application compatibility update. Using MBSA, it doesn't care about printer driver and compatibility updates.