I've already seen 2 machines today that all of the sudden started hanging because of MSE. Googled it and anothers are experiencing the same thing 1 person saw 10 machines already today hanging from MSE. The one issue with MSE is it won't let you uninstall from safemode which is retarded
Boot into safe mode and disable inside Msconfig from both the services and the startup. Then reboot into regular mode and uninstall it.
4.16.2014 UPDATE 22.214.171.124 Security Essentials Virus definitions from Microsoft FIXED the Boot / Lock problem with the MS Security Essentials Virus definitions update 126.96.36.199 and 188.8.131.52. If you can not get into your computer because you loaded the Bad Version you may have to load your backup or restore prior to this day 4.15.2014. or You May be able to stop it from loading in Safe Mode. Then once you do the update from the MS Site or MS Security Essentials 'Update Tab' you will get the fixed MS Security Essentials Virus definitions update. Run a Full Scan After the fix. FYI I would Also Make Sure you do regular Backups (Daily or weekly) , I use Acronis True Image it works well. This will help you if Microsoft play's games.