If your network security is or has been upgraded to WPA2 and you are running XP SP2, your OS might not pick up the new security. If you loose connection or cant connect to a WPA2 connection and it only shows it as WPA then you will need to either;
- Upgrade to XP SP3
- Or download Windows Hotfix KB917021 - http://www.microsoft.com/downloads/details.aspx?FamilyID=2726f32f-d52b-4f84-ace8-f7fc20195769&displaylang=en&Hash=dAhqvqQ1GtpvOsvHDe%2b28NkNlK4x9/Ob7BtljPbqEui/mE0XLXc6d0eVW/aq%2bPVuRPKF9QXfRvkhxy%2b5VhfihQ%3d%3d
The latter is much simpler to do :-)
After either of these solutions you will then notice your OS picking up WPA2 and you'll be able to connect.