Your computer may be hacked/infected, so run it in Safe Mode, disable the Startup applications in Start \ Run \ type; msconfig and press Enter... go to Startup tab and uncheck all except for the antivirus, and delete all user Temp files in Start \ Run \ type: %temp% and press Enter, next again in Start\Run type: temp and press Enter, and delete all Windows temp files.
Next restart the computer and again login in Safe Mode this time in Safe Mode with Networking... and next run Online virus-malware scanns... After this you can try loging in normal mode.
10 Best Free Online Virus Scanners
http://www.technobuzz.net/10-best-free-online-virus-and-malware-scanners/
Edit: to logon in Safe Mode; right after the BIOS posts, press F8 and hold, or tap every second till the Safe Mode option list appears on the screen.
Didn't notice it's Windows 8 so instead of Start \ Run \ the Windows Search bar may work as well to find temps and msconfig.