Usually it's the wireless channel. EVERYTHING can affect it. I even heard that flouresant lights can affect it.
HOWEVER, I have noticed with good use of Windows Vista, that it would constantly drop the connection and NOTHING would restore it, other than restarting the computer. Even the "repair" option wouldn't work.
Damn microsoft. ehehe.
Saying that. Windows 7 is starting to do the same thing (almost) with me, only this time, I will have internet access wirelessly, but the wireless list indicates that I'm not connected to any access points. LOL.
So, I really don't know what's up with the Windows line of operating systems.