Hi
What is the best way to be sure that there is no virus?
The best way is to use an Antivirus application and some Anti malware and Trojan applications.
You use already the Norton Antivirus program. This is great. Be sure that this application is always up-to-date.
Additional you could install Adaware. This is freeware tool and it scans your OS for malware.
But the virus or malware is not always a reason for an OS freezing.
Please check your processes which are running in the OS background. You can use additionally the application called “CClean”.
It scans you OS for troubles and repair the registry.
Maybe it helps…