Try Safe Mode pressing F8 when the machine starts-up.. press and hold till Safe Mode options appear on the screen or tap F8 once every 1.5-2 seconds... and select Last Known Good Configuration... if you can logon,
1. Restore the System to a date previous to the problem,
2. and scan your system for virus, spyware/trojans and rootkits. Update your antivirus and scan, download Malwarebytes Anti-Malware and Avira or AVG antirootkit and scan with each application.
If loging on with the Last Known Good Config failed, try the Safe Mode option, and do the Virus, Malware and Rootkit scans.
I would also sugest you delete temporary files, cookies, clean the registry, etc. with CCleaner, and install Startup Manager to remove programs from the startup leaving only the Antivirus and necessary programs. Also uninstall the last few programs you may have installed before the problem started as one of them may have installed a bug in your system.