When your machine is running slow, first thing you should do is check the Task Manager. To invoke the task manager,
– Click Control-Alt-Del.
In XP Home system, it will automatically opens the Task Manager.
In XP Pro system, select "Task Manager" in Control-
In Vista Machines, select "Start Task Manager" in control-Alt-Del screen.
– OR Right-Click on the task bar and select "Task Manager"
Check which process is taking more CPU time in percentage. Now the real issue is, what do you know about the processes listed in Task Manager. Some times its really hard to identify which process is related to which application on the machine. Or which process is real system/OS related process.
I usually do a hard disk search to find the Process name to see any application under Program Files show up.
Here is the great TIP for you. Head over to http://www.processlibrary.com and search your unknown process that takes too much processor time or memory.