Hi 4j1a,
The "Arduino-1.5.3" should be in the root directory of your C drive due to a know issue with pathnames too long. Another thing to check is that the right port is selected or try switching to a different USB port. If the port is selected correctly and still fails. Try restarting your computer and Intel Galileo, the USB port might have been locked by a process.
The forward and backward slashes on your path should not to be the problem as if i disconnect my Galileo and try to upload i get the same error as you.
Regards,
Intel_Jesus