My 6yo Desktop must be nearing the end of it's life as something is wrong.

When I start it up, all is fine. I can then load one application eg gmail or a web site saved to desk top. When I have finished that, I can no longer open anything from my desktop, nor can I open Google to try another approach.

I can go to the Start menu and close or restart the PC without delay or problems.

Any suggestions ? Have I cooked the memory ? got a Virus ? (I use up to date Norton 2011) or is it me ?