strMinimum=1048576000  'size in bytes (1GB)
strDrive="C:"
If HasFree(strDrive,strMinimum) Then
    WScript.Echo "Drive " & strDrive &_
     " has enough free avaialable space to continue."
Else
    WScript.Echo "Oops! Drive " & strDrive &_
     " does NOT have enough free avaialable space to continue."
    WScript.quit
End If

'run additional code here now that you know the drive has enough
'free available Space

Wscript.quit

Function HasFree(strDrive,strMinimum)
'StrDrive is a drive letter like C:
'strMinimum is the minimum number of bytes to have
'available
Set objFSO=CreateObject("Scripting.FileSystemObject")
Set objdrive=objFSO.GetDrive(strDrive)
If objdrive.AvailableSpace >=strMinimum Then
    HasFree=True
Else
    HasFree=False
End If

End Function