First thing I would check is your network settings. The way to do this varies by operating system:
XP: Start - Control Panel - Network Connections - Right Click on Wireless Network entry - Choose Properties
Vista: Start - Right Click on Network - Properties - Click on Manage Network Connections - Right Click on Wireless Network entry - Choose Properties
7: Start - Right Click on Network - Properties - Network and Sharing Center - Click Change Adapter Settings in the top left - Right Click on Wireless Network entry - Choose Properties
Just make sure adapter has at least the IP v4 and Client for Microsoft Networks protocols and services installed. You may go as far as to add in the IP v6, QoS Packet Scheduler, and the LLTD (Link Layer Topology Detector) protocols as well, although they shouldn't be necessary. It couldn't hurt though.
Next, click on IP v4 and click Properties. From there, check to make sure DHCP settings match what is set on your router (i.e. if your router is set up to be a DHCP server, then leave your settings here on Automatically Obtain IP Address). Also make sure you don't have a default gateway manually entered.
If everything in your Wireless Adapter Properties matches what I mentioned above, you either may have advanced PC issues, conflicting settings, or possibly malware. Check those settings first. If you suspect malware, please direct your concern to the moderator to have your post moved to the correct thread.
Edited by Eric ~ Computer Guy, 30 November 2009 - 10:56 PM.