Well... even if the UDF didn't have a recursion problem, this code would never work.
Code:
$origen="c:\","d:\"
For Each $dr In $origen
   $FileExt=".doc;.xls;.ppt;.zip;.nsf"
   $fulllist=DirPlus($dr,$FileExt,1)
Next


Every iteration of the For Next loop would overwrite $fulllist from the previous iteration.

You might want to give Jens' DirList() UDF a go but you would still have to rethink the above issue.
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