I did a system reset after having deleted LG home on my LG android phone p500. I thought the system app would be restored, but the fact is now. I don't have a "desktop". I cant get to anywhere now on my phone, it is just one big black screen. Anyway that I could fix it? Thanks a million.
Your best bet would be to reflash the ROM, since system reset only clears the user data, it won't restore system apps. A quick google search revealed this tutorial on how to reflash stock ROM: How to flash Original Firmware on the LG Optimus One P500
As long as your Google account still is configured on the device (and the latter is connected to either WiFi or the mobile network), the simplest solution would be to visit the Google Playstore, log in with the same account you're using on your Android device, chose an alternative launcher app (e.g. Holo Launcher), hit the "Install" button, and approve its permissions. A few minutes later, the app should be installed on your device -- and you again have a working home-screen.
There's a big mis-conception about factory-reset, as the name is misleading: It won't reset to "as shipped from the factory", but simply remove all settings and user-installed apps -- so it looks like "fresh from a factory". For details, see the factory-reset tag wiki.