outcoming electronic component with 20 legs, which is called "ATMEGA328P-PU DIP-20 Microcontroller"
the 328p-pu has 28 legs. count em...
The 7805 of the Arduino...
a 7805 is not a standard part for the Uno, your wiring is not very clear in your description. Are you referring to the arduino's on-board regulator or an external component? If the onboard regulator, then it is irrelevant because you are powering the uno from USB. The v. reg. and USB are two seperate power sources.
The middle leg (pin #2) of a 78xx is ground (0v), the left leg (pin#1) is input ( you never mentioned what the voltage is of your external power source you are using), and the right leg (pin #3) is output (5v). If this statement-
... gives me the same Voltage, on the middle leg I get 4.3V and on the left leg I get 3.8V.
- is true, then something is either wrong with how you measured it or the 7805 is damaged or incorrectly wired.
Wiring and description aside, you might try a loop back test (http://arduino.cc/forum/index.php/topic,73748.0.html) to see if the usb-to-serial chip is damaged.