Sony Xperia X10 is boasting of specifications, but most  websites or blog are displaying those specifications, that I will try not to post, but rather on how to reset it to factory defaults. Doing such will make the phone brand new (firmware wise), because all your date will be lost.

I. When the phone is still accessible, go to Settings >> Privacy >> Factory Reset. You have the option to erase everything on the phone and by checking the box for SD card, it will reformat the SD card too. Not unless you dont want to reformat the SD card, dont tick the box.
II. If the phone is not accessible anymore, turn off mobile, remove battery and put it back, press and hold the left and right buttons at the bottom part of the phone then press the power button. Follow the instructions that follows. This is how to hard reset your phone.