Port forwarding is described on page 6-1 of the MR814 manual, found here: ftp://downloads.netgear.com/files/Fu...al_MR814v2.pdf
You want to set up some ports (at least port 80 for HTTP, as in the example in the manual) to be forwarded to your Linux machine, and possibly some to your Windows 2000 machine. Each port can be forwarded to only one machine.
If you already have the port forwarding set up as described in the manual, check that the two machines have different IP addresses.
If they have diffferent IP addresses, run:
tcpdump tcp port 80
as root on your Slackware box. This will print out descriptions of every tcp packet that hits port 80. You can see the packets when the Windows machine is off; this will let you know if they are getting through the router when the Windows machine is on.
If you need to see more detail on the packets, try Ethereal, found at http://ethereal.com.
There are versions for Windows and Linux. (Hell, try Ethereal either way-- it's sweet.)