avrdude -p attiny2313 -P com2 -c stk500 -b 19200 -U flash:w:main.hex
avrdude: stk500_2_ReceiveMessage(): timeout
that 2 in the error message is a strong indicator that you're running the stk500v2 protocol rather than the stk500v1 protocol that is implemented by most of the arduino bootloaders. That's one of the reasons that "-c arduino" was added. If you MUST use the avrdude that came with winavr (instead of, say, copying the avrdude that came with arduino over to your winavr bin directory), try specifying "-c stk500v1"