First day installing Arduino Uno AT328P and that’s the message I got. I’m using windows XP and the UNO board is Chinese, the one with the 2 push buttons in the middle of the board marked "KEY H (high) and KEY L. I got the error message because no Arduino port was established during install. The problem was that Windows didn’t recognize the serial driver which, on my board is WCH CH340T (some have a G). Be sure to look closely at your own.I found the manufacturer in china. Go here, (CH341SER.EXE - 南京沁恒微电子股份有限公司). The site is in chinese but windows will translate it for you. Download the .exe file and then install it. After I did that the problem was completely solved. Be sure you did a CLEAN install of Arduino before running the 340/341 download. Wishing you well.
Not In Sync resp=0x00 prior to installing the drivers indicates that either you already had the drivers installed (and something else changed that made it work) or you had a different serial port selected from tools -> port (so it was trying to talk to some other device - probably just an on-board serial port not connected to anything - not the arduino)
If there was no driver, there wouldn't be a COM port for the computer to talk to the board through, and it would give a different error (about being unable to access some COM port, probably COM1) - unless there was another serial port connected to the computer, in which case it would have been trying to use that one (which presumably wasn't connected to anything).
That error in general (Not In Sync resp=00 or resp=FF ) is very generic - basically any problem with uploading will result in an error like that. You'd get the same error if you cut the board in half, or selected a serial port that wasn't connected to anything, or the chip wasn't bootloaded, or there was some other hardware problem.