After helping a neighbor securing his network, one of his computers was suddenly not able to access either internet nor the router. Note that it was possible to connect to the network and obtain an IP-address via DHCP, but nothing else worked.
When disabling encryption, everything worked fine once again. Also tried both WPA and WPA2 settings in both router and on the computer, with no success. Another computer was able to connect without any issues, I'm quite sure the passkey is correct, and there is no active MAC filter.
The computer which could not connect ran Windows Vista, the router was a Linksys/Cisco E1000. I cannot think of anything other than a driver or network card hardware issue, but is there anything else I may have forgotten?