2K Adv Server, Firewalling Messenger service on external NIC

I am recieving messenger spam on my 2k adv server which is used as a multi-homed router.
i cant believe people will exploit stupid stuff like that to advertise a product that no one will buy. like as if this method of advertising will result in more sales than commercial advertising.

anyways, i now need to seup a firewall on the external nic only to block the messenger service...
i do use the messenger service for error reporting so i do not want to disable the service.
this router has lots of traffic running through all kinds of ports that shift numbers all the time, so i have to be carefull to only disable ports specifically related to messenger. for now im going to follow the following excerpt i found:

Blocking Network Access to the Messenger Service
Blocking access to the service is complicated because it can communicate over multiple protocols, and it shares a port mapper with other applications. Blocking all the possible ports will disable the ability of other computers to send you messages, but it will also disable other services. The most common service that may be affected is Windows file sharing. If you want to share a folder on your computer to the network, this ability may be affected. If you don't want to share a folder across your network, blocking these ports is suggested as a way to improve overall security:

Block access to ports 135, 137-139, and 445. The default configuration of the Internet Connection Firewall shipped with Windows XP will block these ports. Windows NT, 2000, and XP TCP/IP security and filtering options in the network control panel can also be used to block ports. If you have a personal firewall (like BlackICE or ZoneAlarm) you can configure it to block inbound traffic on those ports.

Possible issues with blocking Messenger ports:
- Microsoft Outlook clients can talk to Microsoft Exchange servers on TCP 135
- Windows file sharing requires TCP 139 or 445 depending upon OS
- Server operators, managed networks, and people with custom applications should take great care with blocking ports. Domains and trusts require several of these ports for authentication and other things.
- Some third party applications, particularly management oriented ones may require TCP 135
- Windows Media Technology (also known as NetShow) uses TCP 135 for the Windows Media Administrator and Windows Media Encoder
- According to Microsoft, "Microsoft Office suite and other applications are DCOM aware. You may disable functionality that is in use by blocking ports."
- UDP 137 is needed for netbios name resolution. It and port 138 may be needed for access to netbios resources on the network.
- Some RPC based services exist on high ports (those greater than 1024). It may be possible that those services can be accessed and exploited directly bypassing the mapper on 135.
---------------------------------------------------------------------------------------------------

so looks like ports 135 137 138 139 and 445 are the ones i need to disable unless someone knows more.
i need help with firewalling inbound connections on the external NIC only.
i do not want to install any third party firewall software unless absolutely necessary, and i would like some step by step help.

server details -
2k Server on a 400mhz AMD pc with 64mb ram - Woohoo! smokin machine!
10mbps external NIC - 100mbps internal nic
using routing and remote access, no filtering setup at all
only protocol bound to external NIC = TCP/IP
internal NIC has bound = TCP/IP & IPX/SPX
internal clients configured by DHCP