*** SOLUTION ***

1.- find subkey of file that doesn't open using double click:

1.1- find extension you have conflict in HKEY_CLASSES_ROOT\ (ie: HKEY_CLASSES_ROOT\.xlxs) and then read data of (Default) value (ie: Excel.Sheet.12)

1.2.- find that value again in HKEY_CLASSES_ROOT\ (ie: HKEY_CLASSES_ROOT\Excel.Sheet.12) and look inside subkey shell\open (ie: HKEY_CLASSES_ROOT\Excel.Sheet.12\shell\open)

NOTE: if you need, check http://www.justanswer.com/questions/1u4sk-vista-excel-2007-cannot-open for more on this


2.- disable DDE through:

2.1- rename original ddeexcec key to ddeexec.bk (ie: original HKEY_CLASSES_ROOT\Excel.Sheet.12\shell\open\ddeexe c gets named HKEY_CLASSES_ROOT\Excel.Sheet.12\shell\open\ddeexe c.bk)

WARNING: in next step (2.2) be sure to do not rename command key; command key and command value are not the same things (keys are like folders and values mostly like files inside folders)

2.2- first, look inside comman subkey (ie: HKEY_CLASSES_ROOT\Excel.Sheet.12\shell\open\comman d), then rename command value to command.bk (ie: HKEY_CLASSES_ROOT\Excel.Sheet.12\shell\open\comman d\command gets named HKEY_CLASSES_ROOT\Excel.Sheet.12\shell\open\comman d\command.bk)


3.- be sure correct non DDE command is set as data of (Default) value in command subkey:

3.1 Verify data of (Default) value match this pattern: "...excel.exe" "%1" /e (ie: the value which data was verified is: HKEY_CLASSES_ROOT\Excel.Sheet.12\shell\open\comman d\(Default))

3. 2 If data verified do not match, then edit it in order to match the pattern. (ie: data of HKEY_CLASSES_ROOT\Excel.Sheet.12\shell\open\comman d\(Default) was setted to: "C:\Program Files\Microsoft Office\Office12\EXCEL.EXE" "%1" /e)