Try Safe Mode... restart the computer, and right after the BIOS post, press F8 about every second or press and hold the key till the Safe Mode options list appears on the screen, and select the Safe Mode with Networking line, and press Enter.
Once you've loged on, start your web browser click on the following link, or go to Google.com, type "Online Virus Scan and free removal" and double click on any of the online virus scan websites and to start a virus scan. If you don't solve your problem on the first scan, repeat on another Online antivirus website.
If the online virus scan helps, update your installed antivirus and do a local virus scan.
From here on it's optional so ignore this if you find it complicated:
You can do the following right after you logon, or while you're doing an online virus scan.
Click Start button\click Run\type "msconfig" (no quotes) and press Enter.. \on the Startup tab, uncheck all programs that are checked except for the antivirus... next click on the Services tab, check the "Hide All Microsoft Services" checkbox, and uncheck the ramaining services except for the Antivirus service/s.
Next go to Start\Run, type "%temp%" (without quotes) and press Enter.. this opens your user account temporal files folder, so delete all the files you find in it. Viruses can be lurking on that folder so, it's a good idea to delete the accumulated files occasionally... besides it's good maintenance practice to delete these files on a regular basis. Except for the temp files being used in the present user login, most are leftover from past logins and are completely useless... and should be deleted because Windows doesn't delete them and they can accumulate by the thousands and takeup gigs of valuable disk space... and some can even cause system instability.
Next go to the System Temporal files folder in C:\WINDOWS\Temp and delete all the files.