From http://www.w3schools.com (Copyright Refsnes Data)

ASP AvailableSpace Property

The AvailableSpace property returns the amount of available space to a user on the specified drive or network share.

Note: The value returned is often equal to the value returned by the FreeSpace property.

Syntax

DriveObject.AvailableSpace

Example

<%
Dim fs,d
Set fs=Server.CreateObject("Scripting.FileSystemObject")
Set d=fs.GetDrive("c:")
Response.Write("Drive " & d)
Response.Write(" Available space in bytes: " & d.AvailableSpace)
set d=nothing
set fs=nothing
%>

Output:

Drive c: Available space in bytes: 884465152

Complete Drive Object Reference