The only problem I see with the virus scanner on the firewall box... is:

In order to scan the virus, you have to have the file stored in memory or on the hard disk. Since that box would just be routing the traffic, it wouldn't catch the virus. It would have to download the file to memory or hard disk in order to scan it. Then pass it on to the requester after it has been scanned.

For home, you'll probably be better off installing the av client and have it autoupdate.

I read about (and posted) a solution that would catch the virus in transit... but I'll have to find the post. I'll see if I can find it then edit this.

EDIT: Found it. check out this thread.

But... if this only runs on windows, then you're going to drive the cost of the box up. You're going to need better hardware, pay for another OS, then pay for the firewall and then the AV software. If you do it client based... you can use older hardware for the router and don't have to pay for an OS... just the av software for the clients.