diff --git a/constructor/nsis/main.nsi.tmpl b/constructor/nsis/main.nsi.tmpl
index a2ae2fb2..886891a0 100644
--- a/constructor/nsis/main.nsi.tmpl
+++ b/constructor/nsis/main.nsi.tmpl
@@ -475,6 +475,68 @@ Function InstModePage_Leave
     Pop $0
 FunctionEnd
 
+Function CheckInstdirValidity
+    Push $0
+    Push $1
+    Push $2
+    Push $3
+
+    # Resolve INSTDIR
+    GetFullPathName $0 $INSTDIR
+    # If the directory does not exist or cannot be resolved, $0 will be empty
+    StrCmp $0 "" invalid_dir
+    StrCpy $INSTDIR $0
+
+    # Never run the un/installer when $INSTDIR points at system-critical directories
+
+    StrLen $InstDirLen $INSTDIR
+    # INSTDIR is a full path and has no trailing backslash,
+    # so if its length is 2, it is pointed at a system root
+    StrCmp $InstdirLen 2 invalid_dir
+
+    # Never delete anything inside Windows
+    StrCpy $0 $INSTDIR 7 3
+    StrCmp $0 "Windows" invalid_dir
+
+    StrCpy $0 "ALLUSERSPROFILE APPDATA LOCALAPPDATA PROGRAMDATA PROGRAMFILES PROGRAMFILES(x86) PUBLIC SYSTEMDRIVE SYSTEMROOT USERPROFILE"
+    StrCpy $1 1
+    loop_critical:
+        ${WordFind} $0 " " "E+$1" $2
+        IfErrors endloop_critical
+        ReadEnvStr $3 $2
+        StrCmp $3 $INSTDIR invalid_dir
+        IntOp $1 $1 + 1
+        goto loop_critical
+    endloop_critical:
+
+    # Primitive check to see that $INSTDIR points to the right conda directory
+    StrCpy $0 "_conda.exe conda-meta\history"
+    StrCpy $1 1
+    loop_conda:
+        ${WordFind} $0 " " "E+$1" $2
+        IfErrors endloop_conda
+        IfFileExists $INSTDIR\$2 0 invalid_dir
+        IntOp $1 $1 + 1
+        goto loop_conda
+    endloop_conda:
+
+    # All checks have passed
+    goto valid_dir
+
+    invalid_dir:
+        MessageBox MB_OK|MB_ICONSTOP \
+            "Error: $INSTDIR is not a valid conda directory or another conda directory.$\n\
+             Please run the installer from a conda directory." \
+            /SD IDABORT
+        abort
+    valid_dir:
+
+    Pop $3
+    Pop $2
+    Pop $1
+    Pop $0
+FunctionEnd
+
 Function .onInit
     ${LogSet} on
     Push $0
@@ -844,12 +906,20 @@ Pop $0
 Function OnDirectoryLeave
     ${LogSet} on
     ${If} ${IsNonEmptyDirectory} "$InstDir"
-        DetailPrint "::error:: Directory '$INSTDIR' is not empty, please choose a different location."
-        MessageBox MB_OK|MB_ICONEXCLAMATION \
+        DetailPrint "::error:: Directory '$INSTDIR' is not empty."
+
+        MessageBox MB_YESNO \
             "Directory '$INSTDIR' is not empty,$\n\
-             please choose a different location." \
-            /SD IDOK
-        Abort
+             do you want to update the installation ?" \
+            /SD IDYES \
+            IDYES confirmed_yes IDNO confirmed_no
+        confirmed_no:
+            MessageBox MB_OK|MB_ICONSTOP "Choose another directory." /SD IDOK
+                SetOverwrite "off"
+            Abort
+        confirmed_yes:
+                call CheckInstdirValidity
+                SetOverwrite "ifdiff"
     ${EndIf}
 
     ReadRegStr $LongPathsEnabled HKLM "SYSTEM\CurrentControlSet\Control\FileSystem" "LongPathsEnabled"