Cannot connect from Windows Xp to Windows 7

Hellow,

I had a problem to connect from Windows Xp machines to Windows 7.
My Windows 7 machine is a backup server, where everyone can find all files.

Since we upgraded from Windows 2008 R8 to Windows 7, Windows Xp devices can't connect to a folder from this machine.
Only if we restart the backup server, it works for a few days.
After one or two days, the same problem comes back.

Some know a solution?

Regards
Kevin