This area is full of bad advice (try newer drivers when drivers do not wear out!) so let's try something a bit more solid.
1. I am making one big assumption that the machine is as new from the factory for the OS and drivers.
2. It worked before and there was no change to the router firmware.
3. It worked before and the router was changed (firmware or settings.) If so, read and complete the work at http://support.microsoft.com/kb/928233
4. If item 3 does not help, connect with an ethernet cable. It's cheap and effective.
5. If ethernet is not an option try another wifi thing. I use this one a lot because of the price and it works on Windows 7.
Here is my setup:
Laptop: Toshiba Satellite A105-S2716 (5 yrs. old)
Modem: Motorola Surfboard SB6120
Router: Linksys WRT54G (with updated firmware)
My problems started a few months ago. I had to reset my laptop to factory settings because I had no restore points, lost a great deal of stuff, and it was all caused by a few viruses. I took care of those pesky things. Now, my problem is that when I tried to reconnect to home wireless network wirelessly, I was unable to do so. I can connect to the Network, but I get no actual internet connection.
Interestingly enough, when I took my laptop to a hotel, it was able to connect and I was able to get working wi-fi.
I've been looking for a solution for a while now, mainly because I need the laptop for a lot of upcoming things.
Any tips or solutions guys? I can post more information if needed.