Security for Media Center PC

I know the question of security-related software has been raise a lot in these forum, but I think my question is unique to warrant completely new thread. I've recently built a media center pc based on Vista to exclusive to stream videos from my NAS devices. I have everything set up except for security, and was wondering if anti-virus and firewall protection is really necessary, even for a PC, that hardly access the internet. Yes, I do how vulnerable Windows OS's can be just with an active connection, but I already have a firewall built into the router. The main reason keeping me from installing such software is their horrific effects on the start-up time of the PC. Like a DVD player or TV, I want this particular computer to start as fast as possible (say under 45 sec), and with security software installed, that simply isn't possible.

 

put it in standby.

cold boot is dead.