open task manager, select "Performance" tab, and click [Resource Monitor]. let it run and when you notice your cpu usage is high, you should be able to determine what's chewing up your cpu.
here's a link that gives more details on the resource monitor.
I cannot tell what is wrong with my computer but lately when i use it occasionally both cores on my computer will be at 100% for long periods of time and causes everything to perform slowly and makes my machine heat up and sometimes it will even overheat and hibernate. To give you an idea of the specs on my computer it is an HP Pavilion tx2635us which runs Windows Vista Home Premium 64-bit Edition. The processor is an AMD Turion X2 Ultra ZM-82(2.20GHz)and it has 4GB of memory. I use Kaspersky Internet Security 2009 and from the virus scans it does not come up with any malware and when I run the task manager I cannot tell which process is using the cpu the most, but i did notice that there is no SystemIdleProcess in the list. Any help on this issue would be great.