Well, since it worked before the security was applied, it sounds like you have one of about three issues.
- Your encryption key is incorrect.
- You have not been added to the MAC filtering list.
- Your drivers are not updated enough to run advanced encryption.
- We need to know what type of encryption is in use, WEP, WPA, WPA2.
- What is the version and exact patch level of Windows?
- Have you downloaded and installed the latest drivers for your wireless card.
- What are the exact indications of your failure to connect?
- Have you removed ALL stored wireless network profiles and searched for the network?