I am not aware of any setting or third-party program that allows you co choose the order in which the startup programs are run at boot time. If anyone knows, I would be very interested.
That said, any anti-virus program has their internal settings that affect their behavior at boot time. For example my NOD32 is set to do an automatic startup file check at boot time and user logon. One can turn that off and gain maybe a second, maybe few seconds of boot time. But the security drawback is huge, so I would never recommend meddling with that.
Now, the above example makes me think that the anti-virus is being loaded in the very beginning of the boot sequence, otherwise it won't be able to do anything useful at boot time.