Have a look under the menu Processes in Windows Task Manager. This shows you all the processes that are running and how much CPU% they are using. You should be able to spot the culprit(s) there.
100% may be correct, it depends on which applications you have running. Have a look through the process list and close the programs using the most CPU% to see if they are causing the slow down. Sometimes the name on the list is a bit cryptic, but a quick search on Google usually reveals the source.
You can also use the process list to see how much memory is being used by the various processes, which could also be the reason for your slow down.
(You can access Task Manager via CTRL-ALT-DEL.)