Open the task list (either use ALT+CTRL+DELETE or go to Start > Run and enter "taskmgr" (without the quotes.)) and look for processes using 100% of the CPU, if you find one, that may be your problem.
If not, go to the "Performance" tab. Look at the "Commit Charge" values. If either the total or peak is greater than your physically installed memory, add more RAM or quit some programs.