Hmm...In 2000/XP, you can use the command
chkdsk X: where
X: is the Drive Letter of your choice, though this command does lot more than checking Disk Space.
But, there's a Resource Tool Kit is available for Windows 2000, which has a command called
freedisk to show Free Disk space. You have to extract this file related to this command either from CD or from a website. This MAY work in XP too!
You can download Freedisk.exe
here.