Ask the person who set up your LAN for the following information:
SSID - also known as the network name
Encryption type - WEP, WPA, etc.
Encryption key - a loooong string of digits and letters( a hex number) that is the same for the entire network
IP address of the default gateway
Then, take this information and enter/compare it to what is currently in the properties window of your wireless connection. TO find that, go into:
Start Menu>Network Connections>right-click on Wireless Connection>Wireless Networks tab> select network> click Configure.
Note: in the Wireless Networks tab there is a box right at the top that says: "Use Windows to configure my wireless networks". Check that box if your network card has no management utility from the manufacturer. It will enable you to modify the parameters of the connection.
HTH - for more help, post more details, e.g. wireless adapter, network mgmt. utility, how your network is configured etc.