Have you tried reinstalling your wireless drivers (click here) or connecting via an Ethernet cable? Also don't forget to power cycle your router.
The first thing I would recommend is to check it's nothing to do with the power options - open device manager, find your wireless adapter, double click and look through the settings to make sure that there aren't any auto power management settings that are shutting down your card prematurely.
You could also try resetting your ip address, it's unlikely that this is the cause but it's good to clear it off the list (note that this needs to be done when you are connected to the Wi-Fi):
- Open a command prompt
- Type "ipconfig /release" and press enter
- Type "ipconfig /renew" and press enter