the method that worked for me was holding the midia and power keys for 15 seconds. keep holding it after the screen turns black untill you see a screen that tells you to connect to the computer.
i tried changing my region to canada through the control pannel (i have windows 7), and it allowed me to get the update, but then when it was finished, there was no marketplace tab at the main menu screen!!!!!!!!
that method appears to be dangerous, but then i did the midia/power key method (after changing my region back to US), then it said it was updating to 4.3 AGAIN. this time, though, everything looks like its working perfectly. so this seems to be everyone's best bet. this, or just reformatting everything.