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Today, after returning from vacations, I started my QGIS 1.7 and got this terrible message:

enter image description here

Help, anyone?

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Hi, have you tried to update qtcore4.dll? You could have several qtcore4.dlls in your system. Maybe something happened to your enviroment variables and now you're using old qtcore4.dll. I had some similar problems (but can't say what exactly, it has been long time ago) and it have indeed been due to problems with PATH and PYTHONPATH enviroment variables. So for starters -- try to find all qtcore4.dll files in your system – Mykolas Simutis Sep 5 '11 at 10:26
"As Giovanni said: Rename qtcore4.dll in system32 into something else and QGIS will start." Yes, thank you. That helped with my problem in Lightscribe. I note that the renamed "qtcore4.dll" in the windows\system32 folder was placed there by Nokia. What will happen if I call the Nokia software? Will I have to rename it again? Why does a reputed company like Nokia program such a thing? Best regards Michael – user7660 May 21 '12 at 10:05
Thanks underdark, it works! great! alessandro – user21967 Sep 13 '13 at 10:01
Please don't add "thank you" as an answer. Once you have sufficient reputation, you will be able to vote up questions and answers that you found helpful. – Fezter Sep 13 '13 at 11:34
up vote 11 down vote accepted

In Portugal, the citizen card which offers an authentication and digital signature service that works via browsers puts older qt dll's in windows\system32. Renaming these stops these services from running...

You can copy QGIS Qtcore4.dll and QtGui4.dll to the folder where qgis.exe is placed. This works without renaming the older dll's. The reason this works is the search order used by windows to find the required dll's for an executable. First in line is the executable folder. After that it searches \windows\system32, and only after that PATH is searched.

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That's a classic DLL conflict. To resolve it, search for duplicates of the DLL in Windows system folders and rename them to e.g. QtCore4.dll.bak.

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The solution, translated below, is given on

On OSGeo4W installing version:

1. Find path: C:\OSGeo4W\bin
2. Copy all files to location: C:\OSGeo4W\apps\qgis\bin\ 

On Standalone installing version

1. Find path: C:\programs (or program files) \Quantum GIS Wroclaw\bin\
2. Copy all files to:  C:\programs (or program files) \Quantum GIS Wroclaew\apps\qgis\bin\
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On Windows 7 you schould'nt miss to replace the Qt.Core4.dll in the hidden Windows directory C:\Windows\SysWOW64\

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