Password prevents acquiring network address

Runnin two Dell laptops with Belkin wireless router. Has operated perfectly
for last 18 months, but now has become necessary to secure with password.
Problem is as once I have set password all seems well for a short time, then
one computer (always the same one) won't connect and just displays
'acquiring network address.' Eventually I can get it to connect by playing
around for ages, but not necessarily repeating what I did the time before.
Any help much appreciated.