I have an older HP Lap top [app. 7 yrs.] with Windows XP and a Pentium 64 processor in it. Some times when I  am using it it will slow down so much it takes forever to open a page. If I check the processor usage it says 100%. If I close down all windows with nothing running but the desk top and the background programs IE anti virus Spellcheck etc. it still shows 100% CPU usage. If I reboot it goes back to about 4% usage. Could this be a bad processor? Is there any way to check ??  

   If your processor were bad, your computer would not be running at all.
   It sounds to me that the hog is one of those background programs.
   I would suggest closing all of the programs (even the ones running in the background) and as many of the processes as you can to see which one is being the hog.
   Don't overlook Windows.  Windows does run background processes that can consume a lot of CPU.

To see what processes are running and shut some of them down, download Process Explorer, a free download from Microsoft. I found it at:
http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx

The program shows you running processes and allows you to shut them down.