hi there, ive just had the same problem on my laptop with vista! mine wasnt after a update tho!
what you have to do is 1st goto add/remove programs and at the top of the page that opens click "show all updates" then scroll down and find the last update from windows and remove it and re-start and if it was the update hopefully this will sort it!
what i had to do was start again with my router, mine is from aol so what i did was remove all AOL software then on back of router there is a reset button, so reset router then set up the router with a NEW wireless network name!
mine is sorted now and working fine, anymore help feel free getting back to me about it.
good luck,
Jay.