Connection to Wireless Network always breaks down
Could be one of many, many things.  Have you tried altering the security settings on the router?  Are you close to the router?  Strong signal strength?

Have you tried connecting other laptops?  Is the behaviour different?

Try opening your console (/Applications/Utilities/Console) and connecting.  Show the log list and switch to the system log.  Watch it, connect to your wireless network, and wait for it to disconnect.  Look through the log from the time you connected to the time it disconnected, and see if there are any messages which might indicate what's going on.

Your question is not specific enough, yet, for a proper answer.  Give us more to go on.