problems with wireless N

I've got a problem with my wireless setup, I have a linksys WRT300N
router hooked up to gotsky.com satellite system there is one computer
located in the main house about 40 feet away from the router running
wireless, the computer am having problems with is located approximately
300 feet away from the router, the routers located on the second story
of the main house and is positioned so that it is "looking" at the
second computer through a plate glass window, there are dozens of large
fir trees(80 to 90 foot tall) between the House and the second computer,
I am using a linksys wireless-N PCI adapter, model WMP300N with the
remote multiple antenna, I am running WEP 64bit as I am so far out in
the country that there are absolutely no other signals or people on the
net anywhere near me, I am really in the boonies just me and the Bears
anyway my problem is after installing the system it was working great I
had 56% signal at the 300 foot location, signal strength was -79dbm,
noise level was -88dbm and my transmit receive rate was over 200 Mbps
according to the linksys wireless network Monitor. This was all fine up
till four days ago when I accidentally deleted part of the link systems
driver files, totally accidental, I just screwed up, I could not find
the original CD that came with the PCI adapter so I went to link system
on the other computer and downloaded the latest driver{4.100.15.5)
installed these on my computer and reconfigured everything, no problems
I went right back online then I noticed my transmit and receive rate has
dropped to 27 to 40 Mbps, what did I do wrong? Everything is the same
as it was before, is transmit and receive rate dependent on the signal
strength? If it is and you think it's my problem is there any place I
can get a better antenna for this system? I've looked around the net
and I can't find any for the wireless N PCI adapter I have as it has
three antennas and three co-wax cables going to the adapter, I was
wondering if I could get a better signal if I took this antenna and
mounted it outside maybe put it in a plastic waterproof housing or
something or mount it at the focal point of a parabolic dish. Anyone
ever done this? I'm thinking about trying this just for grins. Thanks
in advance for any replies