If Hitman says it's clear and Norton says it's clear when you scan with it, then in all likelyhood, it's clear.
I can't say for sure whether it's Norton that's causing this freezing problem, only that Norton is notorious for creating as many issues as it fixes.
Microsoft Security Essentials
http://www.microsoft.com/Security_Essentials/ is an antivirus / antimalware package like Norton. It runs constantly in the background, so you'll have to choose between it and Norton if you consider changing. It has a much more basic interface compared to Norton, and uses very little system memory when it's running - both aspects that it has been praised for.
There is nothing stopping you from uninstalling Norton now, trying MSE and, if you don't like it or still have problems, going back to Norton. Just make sure that you write down your Norton subscription code so you can reactivate it if you decide you want it back. Also, to properly uninstall Norton, you need the Norton Removal tool which you can search for and download from their website. Otherwise it won't uninstall correctly and could cause further issues.
It's only my opinion, but I do think that MSE is a fantastic piece of software considering that it's free. I have installed it on both of the PCs I've got running at home and haven't had any problems with it at all. A real turnaround for a company like Microsoft.