If ya patch it often and good, and add a couple of third party addons, for scanning http/ftp traficc for viri, I think Microsoft for once realeased an excellent product in Microsoft ISA Server ( Internet & Acceleration server ).

I use it as The 0nly point of access at our corporation, and it's superb, once u get used to the interface (may seem a bit confusing the first time u look at it).
I'm becoming a devil @ packet filters

And it's pretty cheap compared to other solutions out there....

Cl0x