This copy/pasted from the Illustrator/Windows forum at adobe.com
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Usually the "Cannot run" error is generated by either a missing or
corrupt Adobereg.db file. The Adobereg.db or Adobe Registration Database
file holds information like Serial numbers Illustrator needs to check
when before it launches.
Have you tried trashing your aiprefs file? Have you tried an
uninstall/re-install?
Otherwise the "Adobe Illustrator cannot run." message is generated when
an expiring or beta serial number is being used. You may have a trial
version which expires. If you were on the Illustrator beta, you'll need
to reinstall using a serial number that comes with the actual product.
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There's also a techdoc at adobe
http://www.adobe.com/support/techdocs/15872.htm