$string = "1. Computer name:DC-G11-FTW.contoso.com"
$string.Substring($string.IndexOf(":")+1,$string.length-$string.IndexOf(":")-1)
Uses a substring and IndexOf to break apart the string
PS C:\> $string = "1. Computer name:DC-G11-FTW.contoso.com" >> $string.Substring($string.IndexOf(":")+1,$string.length-$string.IndexOf(":")-1) DC-G11-FTW.contoso.com