I think I'd start with the basics. Try running a full scan with your updated anti-virus suite. No AV product will be 100% effective 100% of the time (if there was such a thing we'd all be using it) so I'd also run a couple of free on-demand scanners.
Malwarebytes ESET Online Scanner
If the problem just recently started you could try restoring your computer to a date/time prior to when the problem began.
System Restore
I would also check for any damaged or corrupt system files by running a system file checker scan from an elevated command prompt (option two.) If problems are found, run the scan 3 times and reboot the computer after each scan.
SFC /SCANNOW Command - System File Checker
You could also check for any application conflicts by performing a clean startup (AKA clean boot.)
Troubleshoot Application Conflicts by Performing a Clean Startup
And you could try running a startup repair. It's been suggested to run the repair 3 times and to reboot the computer after each scan.
Startup Repair
If the problem hasn't been solved after all of this, one of the Forum experts perpared a troubleshooting tutorial with some addition suggestions, including diagnostic checks for hardware issues (failing HDD, RAM, etc.)
Troubleshooting Steps for Windows 7