You can run a service under any Windows user account, but typically Local System is used. For security reasons, this is a limited account which can not access network shared drives. To resolve this problem, you may need to run the service under another Windows user account that has security access to the shares.

Also, even when using a Windows Account, you may not be able to access mapped drive letters. For example, you may have the share \\server\share mapped as the X:\ drive. Since the service does not login to a console session, drive mappings are not automatically restored. You will be able to access the shares using the UNC syntax \\server\share, however.