Sony Ericsson Z800 hard reset via firmware updateDownload PC companion and install
Install PC companion on your PC computer. Follow instruction. For firmware update you need a USB cable.
After your phone connected press Start / in Support Zone
You can also use Sony Ericsson update service tool
PC Suite is out of date use PC companion software
If nothing works: As last resort do a Sony Ericsson phone repaire with PC Suite. After installation Select ----> TOOLS > PHONE REPAIR
Download PC Companion for your phone