I believe the file you are looking for is "...\Documents and Settings\All Users\Application Data\Autodesk\Adlm\ACD2010en_USRegEmail.xml." If you are browsing in to the product folder you will not find this file. If you cannot find this information there is nothing to be concerned about as you can reactivate without it and may have a new request code requiring a new activation codeif you reinstall and remove the license.
This will not return the request and activation codes used but if you are interested in inventorying your install base I'd suggest you use the new Autodesk Asset Locator. This tool replaces the Autodesk Product Manager and enables you to search a network to find installed Autodesk products and identify their locations.
You will find a posting from Tom Stoeckel on the Without a Net blog. Tom has included a link to the Autodesk Asset Locator web page where you can download and find documentation for this tool. This tool is included on the AutoCAD 2011 media or download.
Maybe in a reinstall the files were cleared but the license remained. This would not require a reactivation and would therefore not recreate this file. Otherwise I cannot say for certain; just that it is apparently not there.
Douglas Lauritsen, Autodesk Product Support
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