If You have a Windows 2008 server (not R2)
with the backup wizard, you can not make a backup to a network share,
the solution is easy, install backup command line tools and you can do a backup to a network share at this way:

Wbadmin start backup -backupTarget:\\”server”\”share” -include:C: -vssFull -quiet

replacing “server” and “share” with the appropiate.