"In the Run box, type "Rundll32.exe advapi32.dll,ProcessIdleTasks". This frees up any idle tasks running in the background so that Windows XP can devote its full attention to what you want it to do. For example playing graphic intensive games."
While it DOES free up idle tasks, according to Microsoft, it can take up to 15 minutes to do so. You would not want to run this task before playing a game or using your machine as it will actually cause any tasks waiting for the system to become idle to be performed immediately.
Quote from Microsoft:
"When called from the command line, the ProcessIdleTasks work is done in the background asynchronously. It can take 10 to 15 minutes for idle tasks to complete. Task Manager will report processes running, and the disk will likely be active during this time"
The complete article can be found here:
Note: As the name suggests Idle Tasks are performed only when the computer is idle, anyways.