#3 Use WPA2 Encryption Only
The wireless N standard doesn't support WEP, so connections using this type of encryption will be limited to wireless G at 54Mbps, even when using N gear.
Plus, the first WPA version doesn't even provide maximum wireless performance on N networks. Therefore, you should use WPA2 (with AES) personal or enterprise encryption.
If you still have a very old adapter that supports only WEP encryption, check on the manufacturer's web site for any driver updates, and make sure that Windows is up-to-date.
If that doesn't help, you should trade it out for a new wireless card.
To change the encryption settings, log on to the router's configuration utility by typing its IP address into a browser.
The default IP and the password should be listed in the documentation, if needed. Then find the wireless settings and change the encryption type.