what was the prevailing wireless link speed - 802.11g tends to hop about so I fix mine at 18 or 24M so it doesn't waste its time oscillating between 36, 48 and 54M.

I'm back! I now have a new desktop computer set up in a room about 30 feet and 2 walls from the router. It is using the same LInksys rangebooster USB adapter. Plugged into the laptop in this room, I can achieve a "very good" signal strength with a network speed of 54mb and download speed of about 6.3mb. Plugged into the desktop however, in the same location in the room, the signal strength fluctuates between "low" and "very good", and the network speed between 11mb and 54mb, with seemingly no correlation between the two. Download speed tests show about 0.6mb to 1.5mb, which is massively lower than on the laptop. I have used DrTCP to up the RWIN to 75920 like the laptop, but unfortunately it didn't work this time. Any other ideas please?

is the USB thing literally in the same place - does the body of the PC get between the USB thing and the router ? Different OS ?

Yeah, exactly the same place. If I move it very slightly the signal is much stronger and more consistent and I can achieve 5mb. If someone walks past the hallway the signal drops almost completely. Both laptop and PC have Windows XP SP2.

It's in what I can find to be it's optimum position, and the signal strength stays at "very good", but the network connection speed still varies between 11mb, 12mb, 24mb, 48mb and 54mb, and it's only on the latter 2 that I get decent broadband speed test results, but it fluctuates so often and easily (e.g. someone walking past).

If you are still determined to get better speed you might possibly find one of these useful here or here or here

Worth having a look anyway!

I managed to find an optimised position for the usb adapter and have been enjoying 6.6mbps on speed tests over the last few days.

However, tonight whenever I do a speed test it does what it was doing before - the network speed plummets from 54mb to 11mb whilst doing a speed test and I end up getting 0.6mb as a result from the test. I don't understand why it does this even though the signal strength is still high and the adapter hasn't moved. Any ideas please? RWIN is 131k ish.

other transmitter on same or overlapping channel nearby ? wi-fi works by collision avoidance so someone else on the same channel means less for you.