|Start with quapo's idea of the malware scan. If that still fails to resolve the issue I personally would download Avast Free anti virus, remove Microsft security essentials, if it does not remove cleanly, use the removal tool. You should be asked to restart, upon restart I would install Avast, update it, register it (it's free). Open the avast interface and under Scan Computer, click "boot time", which will take you to another menu allowing you to "schedule a boot time scan", schedule on and restart the computer. It will appear to be restarting windows normally, but an avast command line scanner will come up and prompt you to continue a scan. Do one, if it pops up any viruses, you have options, I usually delete them all, but you may quarantine them if you are not sure. See how that goes. links below.|
Removal tool if MSE does not uninstall using "Programs" uninstall in the control panel.