The problem arises in Windows XP while trying to connect to a secured network, often on a D-link router.
While trying to connect a new computer wirelessly, an error comes up stating that "Windows was unable to find a certificate to log you onto the network (Network name here).
Here's how to fix it if it is WEP encryption...
Start -> Programs -> Accessories -> Communications -> Network Connections -> Right-click your Connection and select Properties -> Click on the Authentication tab -> Disable / uncheck the option IEEE 802.1x authentication for this network. Now, try to connect to your Wireless network; are you able to? (Note: you may need to restart the computer.)