Need Help, my ATmega328p is now set at 8Mhz (Internal).

clausismus:
Then I took the ATmega328p out of the UsbTiny and inserted it to the Arduino Duamilanove.
But now the Arduino board won't be detected by my PC.

The state of the ATmega won't affect the FTDI 232RL's ability to be enumerated by the PC. Did you remember to install the FTDI drivers?