Mighty strange behavior

Hello,
My Dell PC runs Windows 7. Almost a week ago, it started behaving strangely. MS Word would not save no matter how many times I clicked the Save button. Same with Notepad.

Then I found that I couldn't use Alt-Tab to change to another open program Window. Also I couldn't access any of the open program buttons on the Taskbar. I had to Alt-Tab-Delete to bring up Task Manager to switch to another open program.

When I tried to search the Net for solutions, I found that after Firefox opened on the homepage, every time I entered an URL and pressed Enter, Firefox seem to try to go to that URL and then returned to the homepage. Only way I could surf is if I opened a link in a new tab. Same thing with Chrome.

In Windows Explorer, after clicking a folder in the left pane to see its contents, nothing happens. When I double-clicked the folder in the left pane, the folder opens up only in the left pane with the right pane remaining unaffected. If I want to open a folder in the right pane, I have to right click the folder and open it in a new window.

I thought that my PC could have a virus, so I updated my Norton Antivirus and then scanned the entire system. After several hours, NAV only found 45 cookies which were removed. No difference. I then scanned my PC on several antivirus websites. No joy. Most found no viruses. One found a virus in an exe file which was deleted. Still no change.

Has anyone experienced such weird behavior? Do I have some virus? Any one has any idea how to return my PC to a working condition? Thanks in advance.