hy detlef,
i think the following key could it be:
HKEY_CLASSES_ROOT\.xlsx
maybe my file association could help. it opens Excel 2010
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\.xlsx]
"Content Type"="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
@="Excel.Sheet.12"
"PerceivedType"="document"
[HKEY_CLASSES_ROOT\.xlsx\Excel.Sheet.12]
[HKEY_CLASSES_ROOT\.xlsx\Excel.Sheet.12\ShellNew]
"FileName"="excel12.xlsx"
[HKEY_CLASSES_ROOT\.xlsx\PersistentHandler]
@="{4887767F-7ADC-4983-B576-88FB643D6F79}"
[HKEY_CLASSES_ROOT\.xlsx\ShellEx]
[HKEY_CLASSES_ROOT\.xlsx\ShellEx\PropertyHandler]
@="{993BE281-6695-4BA5-8A2A-7AACBFAAB69E}"
[HKEY_CLASSES_ROOT\.xlsx\ShellEx\{8895b1c6-b41f-4c1c-a562-0d564250836f}]
@="{00020827-0000-0000-C000-000000000046}"
[HKEY_CLASSES_ROOT\.xlsx\ShellEx\{BB2E617C-0920-11d1-9A0B-00C04FC2D6C1}]
@="{C41662BB-1FA0-4CE0-8DC5-9B7F8279FF97}"
before you do this, please export your old key from your registry. than you wont have trouble to restore
best regards
george