When first switched on my pc takes several minutes to load Windows XP. Then to check e-mail or connect to a website it takes minutes again. However, when it has been on for 15 or 20 minutes it begins to perform at normal speed.
Is it configured to do a virus / spyware scan on booting ?
In any event, you need to look at the Task Manager (Ctrl-Alt-Delete or right click on taskbar) to see which processes are monopolising the CPU. In task manager click the Processes tab, then clicking on the CPU column will toggle between low/high users of CPU.