I just tried on Windows 7 64bits OS and it had no problem to obtain IP address via DHCP.
What does the error message say?
You mentioned that XP and 32bit hosts work as expected, but not Win 7 64bits. This means that the VPN configurations on FortiGate probably is fine (assuming the 64bits host uses the exact same tunnel setup on FortiGate). If this is the case, it must be something different on the host. Do you have any other AntiVirus software or other VPN client installed on the 64bits host?
If the host uses a different VPN setup on FortiGate, you need to check the configurations on FortiGate to make sure things are properly configured .
L.Clarke
Ling Lu