[sc00bie]

What mode is the router using? 'g' 'b' or mixed ?
What encryption are you using on the laptop and ibook - try disabling and see how it goes. I think it's 40/128 bit on ibook and 64/128 bit on the PC, try changing this to 128 bit on both.

Are all wireless cards capable of the same e.g 54mbs, are they different makes.
Have you checked the maufacturers web sites for compatability issues with other maufacturers products?
As said before it sounds like the signal produced by the Mac is flooding or is stronger than the card used on the laptop.
If you go to a friends house with wireless access does the same thing happen using the ibook.

Untill you can verify that there is no compatibilty issues between the cards used you will not be able to narrow things down. Changes need to be made one at a time, test, continue.
Have you used the FAQ section for the mac, have others experienced the same problem, would do some digging around the hardware as it is not the routers.
sc00bie