This question already has an answer here:

Why is ArcMap 10.5.1 search window not showing properly?

Out of ca 50 times opening ArcMap, only twice or so it looked like in previous versions, i.e. there was possibility to work with that - e.g. enter a name of function and search for it...

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I am using Windows 7 Professional.

I have
-renamed ESRI folder, also in registry, not helped!
-deleted Normal.mxt, no change!
-chosen "repair" in instalation file window, also not helped!

merged by PolyGeo Mar 14 '18 at 7:10

This question was merged with Search window not opening in ArcMap? because it is an exact duplicate of that question.