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So there is an option to disable this feature called "Ignore Other Applications" (under Options-> General). If you look at the help on this it basically shuts of DDE. What DDE basically does is when you click on a file it checks to see if Excel is already running and if it is, it passes the file to that already running instance. The problem with turning this switch on is the file mappings that are already installed assume DDE is on and things break. So when you shut that option off you suddenly start getting errors that Excel cannot find the
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Open regedit, browse to HKEY_CLASSES_ROOT\Excel.Sheet.8\shell\Open Delete the ddeexec key, (or just rename it if you are worried - i just renamed it and it worked) Then click on the "command" key and replace the /e on the Default and command string in that key with "%1" Quotes around %1 are important. NO QUOTES MEANS NO WORKIE!!!!
HKEY_CLASSES_ROOT\Excel.Sheet.8\shell\Open\command should look like this: (Default) REG_SZ "C:\Program Files (x86)\Microsoft Office\Office12\EXCEL.EXE" "%1 command REG_MULTI_SZ vUpAVX!!!!!!!!!MKKSkEXCELFiles>tW{~$4Q]c@II=l2xaTO5 "%1"
Do the same for Excel.Sheet.12 Now Both .xls and .xlsx should open in new windows with no errors.
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Reference
MS Office is breaking ALT-TAB :
http://www.donationcoder.com/forum/index.php?topic=27996.0;wap2
Getting Excel MDI/SDI/Excel Instance to behave more logically :
http://www.tech-archive.net/Archive/Excel/microsoft.public.excel.setup/2006-02/msg00115.html
Windows 7 - Cannot run multiple instances of excel using "Ignore other applications that use DDE" option :
http://social.technet.microsoft.com/forums/en-US/officesetupdeploylegacy/thread/c42f825d-b7e8-48c8-b5af-30563b839f09
