When the processor goes to 100% is it from the same program every time? Go into Task Manager and then the Processes tab and sort by Mem Usage and then CPU and see if any process has a really high value. Then lookup that process online if its not obvious what it is by looking at it. You can also right click the process and choose End Process and see if that makes the CPU level go back to normal.
I would also go through your startup items using a href="/support-categories/pc-troubleshooting/188-checking-your-startup-items-using-msconfig" target="blank">MSconfig and uncheck anything that you don't need to boot with your computer. Most things in the list can be safely unchecked. You can lookup the items online if you cant tell what programs the relate to by the name.