Seeing as you have already done scans with a few of the programs regularly recommended here and found nothing, I am guessing PCGuard is generating the warnings and it's an update to that which has failed. Choices are ;
Download again, disconnect from net, uninstall current version, run CCleaner, reboot, install new version.
Get a different free AV; Avast/AVG - click here
Only have one AV installed at any one time though.