In the task manager click on the processes tab. Click on the CPU column twice so the processes using the highest %age of CPU time are at the top. The list should be constantly changing with the System Idle Process at the top in a PC that is running OK. THe offending process will probably be consuming greater than 50% of the CPU time all the time. Make a note of the process name and highlight it and press End Process. If that improves the performance you then need to stop the process starting in the first place. Depending on the process there is a variety of ways of doing this. If it is easily identifiable - stop the program starting, if not it may require a bit of detective work.