Get disk space percentage free with SQL Server 2005?
This can be done through PowerShell.  See the below:

Get-PSDrive |
    Where-Object {$_.Provider -like "*FileSystem"} |
    Select-Object Name, Used, Free, @{Name = "Used_Percent"; expression = {$_.Used / ($_.Used + $_.Free) * 100}}


Or, through WMI:

Get-WmiObject -Class Win32_LogicalDisk |
    Select-Object DeviceID, VolumeName, @{Name = "Used_Percent"; expression = {($_.Size - $_.FreeSpace) / $_.Size * 100}}


Please see Aaron's comment to your question.  I completely agree with him, this is not something that is easily done through SQL Server.  You can finagle this to be run within SQL Server, but it's not necessarily a native way to get this information directly from SQL Server.