Update your installed anti-virus suite and run a full scan. After that run some additional on-demand scans using these free products.
Malwarebytes
ESET Online Scanner
If the scans come back clean I'd look for damaged or corrupt system files. Run a system file checker scan from an elevated command prompt (option two.) If any problems are found, run the scan 3 times and reboo the computer after each scan.
http://www.sevenforums.com/tutorials/1538-sfc-scannow-command-system-file-checker.html
And you might want to go through these troubleshooting guides.
http://www.sevenforums.com/tutorials/220165-troubleshooting-steps-windows-7-a.html
http://www.sevenforums.com/tutorials/219533-troubleshooting-windows-7-failure-boot.html
Please post back if anything helped or not.