OK, then something is limiting your transfer rate. From the specs on your card, the P2 can do 100baseT. It's a bit tricky to get Windoze to disclose the speed and mode (half or full duplex) but you can sorta tell by decoding the lights on the ethernet port and/or on the routers ethernet switch. Sometimes, the status page on the wireless router shows the connection speed. You can also fire up the Windoze Performance Monitor and check the scale on the network traffic. My guess(tm) is that if it's capeable of doing 100Mbit/sec, then it probably is doing 100Mbits/sec.
One interesting test would be to ignore the wireless (by disabling it) and just plug the P4 into the router with a CAT5 ethernet cable. I'm sure the P4 can do 100Mbit/sec sec. Then, try your transfer test. If it still shows 12Mbits/sec, then something is seriously broken, probably in the P2. If you're not sure, try it on a variety of machines, with and without wireless, and see if you can isolate the culprit. It might be helpful to find another fast P4 machine to temporarily replace the P2 and see if it magically fixes the problem.
I use IPerf for most of my benchmarking.
Another interesting and simple test is to just ping one machine from the other via a wireless link using Windoze: ping -t other_machine_ip_address If the ping time in time=msec column varies radically or shows lost packets, you're getting some multipath or interference. It should be a fairly low number (2-3msec) be constant.
Yep. It can do 100Mbit/sec.
I'm not a big fan of the Realtek 8139 chipset, but it does work, is quite common, and should be more than adequate. You might want to dive into: Control Panel -> Network -> Realtek Right click on Realtek and select properties. Dive into the list of device properties and make sure that nobody has accidentally set the speed down to 10Mbits/sec. It should be "auto" or you can try to force it to 100baseT-FDX.
See: