netgear mr814v2 issue

I have a mr814v2 and i have serious problems accessing my dsl over it.
I can get on the net but packets larger than 1400 bytes dont seem to
pass. When i go to yahoo mail it doest work either, i get a document
contains no data error from my browser. I have tried two different
network cards, a linksys wpc11 and the built in broadcom .11g in my
laptop. Both of these were running XP.
Funny thing is it all works great on my mac, same wireless router , 0
issues.
Any ideas?