SQL Server by default installs iteself so that is uses the system account for logon. You have to change this, because the system account does not allow access to network resources. This may also be the cause of errors when a backup cannot be made because a network location is unavailable:

\\Server\Backup\BkpDataLab19991109160205.dat'. Device error or device off-line.
See the SQL Server error log for more details.

You have to change the SQL Service to log on using a domain account. Go to Control Panel | Administrative Tools | Services, find MSSQLSERVER, right click and select Properties. Go to the Log On tab.

Select the option This Account and enter a domain user and the associated password that should be used by SQL Server to access network resources.

The specified domain user must have access to all objects required by SQL Server and read/write permission for the folder where backups are stored. Because of that, a user with administration privileges is often used.

The logon account can also be specified in SQL Server Enterprise Manager.

If you use the Full-Text Search service, you have specify the logon account with it, otherwise the synchornization between SQL Server and Full-Text Search won't work.

If you change the account's password or delete the account, make sure to make the same changes for the SQL Server instance.