LogonUser API Call not working in Windows 7

Question

Hi All,
I am utilizing the LogonUser API to impersonate a user and read the contetnts of a server directory files. The problem is that its working fine in XP.( Client mahines-XP,File Server-Win2008 ) but in clients who have windows 7 have the problem accessing those files through the impersonated user. It's giving access denied error .
is there any thing we can do in the program? or any security policy change in Windows 7 client?
anyone please help
Mee