Get "The specified account does not exist" when try to access file

When I try to access a file in a program using the usual syntax of \\computername\sharename\directory\file on a computer which isn't the domain server, I get "The specified account does not exist". If I then go into Windows Explorer and display \\computername, I can then access the file in other programs. However after a period (don't know how long - maybe an hour), I will get the same error again if I haven't been accessing anything on \\computername. The computer I am accessing is on the domain, I have authority to access the files and I should not be getting this error. This doesn't happen in any other version of Windows - just Vista. Any ideas?