a few domains cant be obtained with belkin wireless router

No security is set up and the rest of the net works but msn instant messenger doesn't work nor hotmail.com and tiscali webmail.

It was all fine before. I don't know where to start troubleshooting this problem.
thanks for a list of things to check. I couldn't work out what to put in the security boxes as numbers, didn't seem to work.

Thanks for replying, I spent some time messing with the values in the routers wizard from other help on the forum noteable that which delt with something like MPUX or MPu value ? its not this machine where the problem is so I can easily give the information.

thanks anyway to you blue.