Help with low throughput?

I have two computers both are Windows XP SP3, both have 512MB RAM both have close to the same processor speed (800MHz and 1000Mhz). On the 1kMhz box it has a LOT of programs, was setup years ago and has not been cleaned off....yet going to speedtest.net I show 20Mbs download speeds. On the 800Mhz box that I recently setup for my kid as a learning box and has nearly nothing installed on it, I am only getting 2Mbs to 3Mbs download speed. I thought maybe it could be a problem in the actual network cable so I even tried swapping cables around yet the problem remains on the newly setup XP box. What could cause such a speed difference between two nearly identical machines?