Like Jeff I use xp_cmdshell with PowerShell to query the Win32_Volume WMI class. You wouldn't need xp_cmdshell or PowerShell at all if SQL could issue WQL command, but alas. This gets both total capacity, label and free space information on both fixed drives and mount points:

xp_cmdshell 'powershell -command "get-wmiobject -class win32_volume | select-object -property name, label, capacity, freespace | sort-object -property name | format-table -wrap"'