I don’t know your router D-Link DIR100 have 2 WAN port or not, if its have 2 WAN port try clustering. Otherwise, install an ISA server with 3 network cards, use 2 cards for input line from ISP and the rest is going to be output to the switch, LAN.
There are two clustering technologies.
The first, called the <b><a href=”http://technet.microsoft.com/en-us/library/bb727117.aspx”>Cluster service</a></b>, is used to link individual servers so they can perform common tasks. If one server stops functioning, its workload is transferred to the other server.
The second clustering technology, called <b><a href=”http://technet.microsoft.com/en-us/library/bb742455.aspx”>Network Load Balancing (NLB)</a></b>, is used to make sure a server is always available to handle requests.