Uninstall BitDefender as suggested. Install Malwarebytes, update, run a full scan.
Intall MS Security Essentials, update, run a full scan and leave it standing guard.
Type CMD in Start Search box, rightclick result>Run As Admin, type "sfc /scanow" in command prompt to check System Files.
Burn to CD memtest86, boot and test memory for 5-6 passes or overnight.
Burn to CD your HD maker's diag/repair scan, run full scan: Hard Drive Diagnostics Tools and Utilities (Storage) - TACKtech Corp.
Then run a Disk Check
Work through repeat errors in Event Viewer>Admin View by googling text and ID# to see how others solved them. Check also Performance Log (on Advanced Tools page accessed by clicking through WEI score link at Computer>Properties) for errors logged at moment of hang. While on Advanced Tools page check for cued issues at top and Generate a System health report.
Consider a Repair Install