I am running in to this same problem about every six months. So far, one of these methods works every time the network connections break again.
To fix certain networking problems in Windows XP:
I have a Lenovo T60p from December 2006. I've re-imaged it two or three times since then, but the last time was a while ago. I've been trying to troubleshoot and fix rather that reformat.
a) A reformat with a fresh, clean, basic install of XP Pro
b) A reformat with Vista?
d) None of the above