Wireless connection drops constantly after installing Leopard.

I have a brand new Intel iMac (the last of the white ones that existed earlier this summer) and my wireless connection keeps dropping. It started happening as soon as I installed Leopard. It's connected to my boyfriend's PC, one he made himself. It constantly drops both our connections after a few hours.

Originally he said it looked like it was caused by Leopard enabling Bluetooth but I turned it off and it still happens... quite a lot. Anyone else having this problem?