The ONLY way to reflash your system is to flash it using the phone built-in flasher.
As already mentioned put the update.app in a 'dload' folder in the root directory of your SD card. Turn off your phone and press Power+Volume up+Volume Down simultaneously. I did that already more then 3 times and it's working perfect!
This will start the flash operation immediately. After its done your phone should work like before.
Ok, will do so. Do I have to download the file mentioned in this thread, the so called "ROM". I will let you know if it works but it will take some time
because the internet over here is pretty slow, takes about 4 hours to download the rom...