We have two laptops (one Apple Mac) & a PC all connecting to the internet wirelessly.
The Mac is fine. The other laptop occasionally comes up with a message saying "work offline" & the fixed PC keeps losing signal & therefore logging out of things like MSN etc. Randomly, the PC is the closest to the router.
This is because Mac wireless connectivity is always excellent, and PC is notoriously rubbish.
If you are running XP on the PC and laptop, make sure that you have all the latest updates installed.
If the PC is in reasonable cabling distance from the router, best to plug it in directly, assuming that it has a network card. Failing that, try disabling the wireless network cards utility and using the Windows wireless zero configuration instead.