How is your router configured? Mine is set to allow three connections at a time - if a fourth tries it shows this message.
There are many possibilities why this is happening - have you tried connecting the computer to the router by cable? Does that help?
A discussion here gives many suggestions:
http://www.webuser.co.uk/forums/showflat.php?N umber=181433
I believe in simple things first. Switch everything off at the mains. Wait at least 30 seconds. Switch back on in this EXACT order:
1. Modem ...wait a few seconds for lights to settle
2. Router (if seperate) ...wait
3. Computers.
If all else fails, reset the router to factory settings and then reset the security.