The thread is confusing. Are you looking for disk space or RAM?

xp_fixeddrives indeed gives you the available amount of free disk space (not the total space).

If you want to know the total amount of free RAM in the machine, I don't think there is a DMV that exposes this, nor is there any reason why there should be. There is little reason why SQL Server would track what else is going on in the machine. But if there is a command-line command to give you the information, you could invoke it through xp_cmdshell.