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Using a plain old batch file for this is going to prove tedious and cumbersome. Instead consider doing your script in something like VBscript (included in XP) or Powershell (probably would require installation). Rough VBscript example:
Dim fso, drv
Set fso = CreateObject("Scripting.FileSystemObject")
Set drv = fso.GetDrive("C:")
IF drv.FreeSpace / (1024 *1024) > 255 Then
'
' Do some useful stuff here
'
wscript.echo drv.FreeSpace / (1024 * 1024)
End If