Wireless router broadband speeds....

Ok, heres the situation:

NTL broadband
2mb
Linksys WRT54g wireless router
2 PC's with wireless cards
Encryption on

Now when just my computer is on and connected to the network I'm getting a download speed on irishisptest of around 1800k - pretty damn good.

Now when the second PC is on and connection to the network, I would assume since the connection is being shared that I should get about half that - in the region of 500 to 900.

But no! I always, always get speeds, on the same test of only about 100k. Between 50 to 100k!!! 10 times slower...What the hell is going on?