Unable to open Excel/word from UNC path

We use Native windows 2000 domain with win2k pro clients.When users login network drives are mapped using logon script.Recently we found that on some machines users are unable to open Excel/word files from UNC path but if same path is mapped to a drive letter they can open it.
For example users unable to open files from \\microsoft.com\support\123.xls but if \\microsoft.com\support is mapped to x:\ or any drive letter they can open it from x:\123.xls.

It works for some machines.On problamatic machines SP1 was installed so we upgraded them to SP2 but no help.We use dfs also.But we need this functionality for one of our application, which uploads xls files to a unc path.