i installed a jar file and when i load it the phone says "starting midlet" and nothing happens, what does this mean?

It means your MIDlet has some problem, or it was writen in a MIDP version with some particularities not supported in the P800 (as is the case with many MIDlets writen for 7650 and alike) or it has some "bug" that prevents it to running, as it happens to me lots of time when i was writing my xjCrono MIDlet.