Ultra slow transfer rates

Hi all!

I've recently setup a small home network (workgroup) between 2 pcs, one running Windows 7 and the other running Windows XP.

Everything is working fine when I share folders or disks, the only thing that is bugging me is the insanely slow rates of transfers, which make it totally unsuable.

I typically transfer around 240Kb/sec :-(
Even my internet is faster.

Here's the hardware specification of the whole thing:
I am using a D-Link DI-624 router.
The whole thing is totally wireless, each PC receiving the router's signal using a USB wireless adapter.

As expected the PC closer to the router gets a better signal than the one further away:
1. the PC running Windows XP, using a D-Link RangeBooster DWA130 N at 54Mbps connection - signal at 90%
2. the PC running Windows 7, using a D-Link RangeBooster WUA-2340 108Mbps connection - signal around 64%

Does any of you have an idea on what's going on? Is it purely due to my router who's not able to cope with it? Is there some software settings I can enable/disable?

Any help greatly appreciated, this is driving me crazy :-(