Good evening. .
I use Firefox but don't use norton or any program for antivirus that runs all the time and is invasive. My PC runs wellHP Pavilion p6110f, 6gb RAM 3 hard drives and it will run picture editing and mediaplayer while broswing without a hitch. Win7 started to shut down programs left and right, change select start back to load em all. Deny access to files so I would have to "repossess" them. I believe I should tell the PC what to do not the other way around.
While the symptoms that you describe could have a number of causes, given that some of them are linked to known infections and that you don't seem to have adequate security installed on your system, I tend to think that the issue is malicious in nature.
While there are certain things that you can do to reduce the infection risks that being online pose, avoiding file sharing and not surfing "the dark side" (adult sites for example) to name but two, I have yet to be convinced by anyone that it is possible to use a Windows operating system that is connected to the internet, with a sufficient degree of confidence in its security, without a resident, active, anti-virus program.
Common-or-garden applications such Adobe Reader and Acrobat and Sun Java, have repeatedly provided access to computers for infections. While you can surf without both, could you say the same for your browser, Firefox? - take a look at the number of critical vulnerabilities that there have been.
The basic level of security that you should have, in my opinion, is a firewall that monitors both inbound and outbound connections (known as a two-way firewall) and a resident anti-virus program. The reason why you shouldn't rely on what are known as on-demand scanners (those that will only work when you initiate a scan) is that they will not actively seek to prevent malicious activity. An on-demand scanner may detect an infection when you have it check your system, but it will be too late to stop it carrying out it's programming, whatever that may be.
In the above quote your last sentence offers me a closing point - if you want to be the one that tells your computer what to do, you need to ensure that somebody else doesn't get there first.