If you get stsadm –o preupgradecheck abort message saying something like
“An error occurred while parsing entity name”
then fear not and go to …\12\CONFIG\PreUpgradeCheck folder and look for the rules files.
Now based on the error message, you can locate which rule went bizzerk. You can either fix your farm setting for example, may be against rule “SPSearchInfo”; you may have defined an external content search entry, where the start address had a "&" sign in the URL the parser couldn't handle OR you can even remove that rule to get other passed results to get things moving.
Hope this helps!
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