Checkpoint [Ithr] Error - Dell dimension

There is really no easy solution for this type of error and there is no master list of error codes that will explain it.   If the system's diagnostic lights are of no help in pin pointing the problem then click here for a Google search on that error code and review some of the sites discussing that problem.   

My friend told me to unplug everything (keyboard, mouse, screeen, power supply), then restart the computer without connecting anything but power supply. And it worked...

Glad to hear you were able to resolve the problem.  Thanks for posting back with an update.