So the reader is aware: This article covers the main causes for this issue, but some of the other Instructable users have found somewhat "niche" causes to the issue that were not covered in this article and are discussed in the comment section below. If this article does not solve your problem, please visit the comment section below.
I have had this error: I would plug my Arduino Uno into my Windows 7 PC (this guide should be relevant for Windows 8, Vista and XP also) but whenever I would upload any sketch to the board, all I would get is
avrdude stk500_getsync() not in sync resp=0x30
for a response. This board however worked seamlessly with my Linux machine.
After much searching online, I found there was no 'clear' solution to this.
Joyfully, the fix is actually very easy (at least for me, hopefully for you as well!). So far, there seems to be 2 3 big causes to this issue:
Unlikely Cause #1: Bootloader is missing
Before we begin, we should check the bootloader of the Arduino for comprehension. If the bootloader is gone or corrupt, this can cause the issue.
To do this: power and reset the board. Check to see if the pin 13 built-in LED lights; if it is not then your bootloader may be missing. If this is the case, then your problem is beyond the scope of this guide. I would suggest visiting this Sparkfun guide: http://www.sparkfun.com/tutorials/247.
If the LED lights, then your bootloader still exists.
Cause #0: Wire coming out of pin 0, (RX)
All credit goes to tvance929 and his comment below for finding this one. Some people seem to have this happen to them so it seems worth it to bring this up top here.
Apparently this issue will also occur if you have a wire connected to pin 0 (RX) on the Arduino.
Cause #1: Wrong COM Port or Wrong Arduino Board selected
With the Arduino plugged to the computer, open "Device Manager" and check the "Ports". Check to see if an Arduino device is listed here; if it is, then make note of which COM port the Arduino is on and check that you have this selected in the Arduino IDE (Tools -> Serial Ports). If the COM port your board is on does not show within the IDE, then try different USB ports.
If you can't even see "Ports"; then for some reason, your computer isn't even recognizing that a device is plugged into it. Test your computer by plugging other USB devices into that port. Possibly your Arduino fails at life; try it with other computers.
It appears some have received this issue by having the incorrect Arduino board selected as cdogjune has pointed out. Check that you have picked the right board in the "Tools" menu.
Cause #2: Drivers not Installed
This is what you should see when you open Device Manager:
If you don't see an Arduino device listed under Ports; then you are probably seeing an "Unknown Device". Check to make sure that "Unknown Device" is indeed, the Arduino by plugging and unplugging the board from the computer. "Unknown Device" should appear and reappear. Assuming this is the case, you will need to install the drivers manually for your Arduino. Windows XP, Vista and 7 users may continue with the guide; if you are a Windows8 user, then please make reference to the comment left by finnigan16 below.
Right click on that "Unknown Device" and select "Properties". Click on the "Driver" Tab. Below, you can see I already have the driver set, hence why my window reads "Arduino UNO R3 (COM4) Properties", when yours will probably read "Unknown Device Properties".
Select to "Update Driver..." and then choose to select the folder containing the driver on your computer. Conveniently, you already have the drivers for the Arduino on your computer when you downloaded the Arduino IDE. Navigate to the Arduino folder you downloaded that contains the Arduino IDE. In that folder, go to the "drivers" folder and choose the "drivers" folder itself (see the picture below for the path). The computer should automatically choose to install one of the drivers within the folder you selected. Simply proceed with the installation and that should fix the problem.
Make sure the Serial Port is then selected correctly!
If you have checked the possible causes discussed here and your issue still isn't solved, be sure to check the comment section. Some of you have found niche causes to the issues which are discussed below.