active sync is suppose to drop when you're flashing.......put your phone in bootloader mode (multicolored screen) by pressing & holding power + camera + soft reset. Now conect to your pc & run the flash again.
and just to be clear, you're not trying to flash a custom rom are you? if so, you need to be unlocked 1st.
Your probably better off flashing a custom rom, follow the instructions in this link to unlock.
http://forum.ppcgeeks.com/showthread.php?t=20370