Start by identifying your exact error. Most QGIS install and startup errors on Windows fall into three categories: installer blocked by security software, missing dependencies, or Python/GDAL version conflicts.
Install errors
Common install errors and fixes
Windows SmartScreen blocked the installer
SmartScreen flags less-known publishers even when signed
Click More info > Run anyway. Safe for official OSGeo installer.OSGeo4W setup fails to download packages
Mirror server timeout or network issue
Retry with a different mirror or check your internet connectionError: "Unable to create directory" during install
Insufficient permissions on Program Files
Right-click osgeo4w-setup.exe > Run as administratorAntivirus quarantines QGIS files during install
Antivirus flags GDAL/Python native DLLs as suspicious
Temporarily disable AV during install, then add QGIS folder to exclusionsStartup errors
QGIS fails to start or crashes on launch
QGIS opens then immediately closes
Corrupted QGIS profile or conflicting plugin
Hold Shift while launching QGIS to disable pluginsError: "The application was unable to start correctly (0xc000007b)"
32-bit / 64-bit DLL mismatch
Reinstall Visual C++ Redistributable x64 from microsoft.comPython error on startup: "No module named qgis"
PYTHONPATH not set correctly after install
Reinstall via OSGeo4W. Do not use system Python with QGIS.QGIS splash screen appears then disappears
Plugin crash during startup
Launch with Shift held to skip plugins, then disable problem pluginQuick diagnostics
Diagnose QGIS issues on Windows
# Check QGIS and Python versions:
>>> import qgis; print(qgis.utils.QGis.QGIS_VERSION)
>>> import sys; print(sys.version)
>>> from osgeo import gdal; print(gdal.__version__)
# Check installed plugins:
>>> import qgis.utils; print(list(qgis.utils.plugins.keys()))
Clean reinstall
Full clean reinstall of QGIS
- 1
Uninstall QGIS
Re-run
osgeo4w-setup.exe→ select packages to uninstall, or use Control Panel → Programs for the MSI version. - 2
Delete QGIS profile (optional)
Delete
%APPDATA%\QGIS\QGIS3to remove all settings, plugins and custom configurations. This is the "nuclear option" — only do this if other fixes have not worked. - 3
Reinstall fresh
Download and run
osgeo4w-setup.exeagain. Choose Express Desktop Install and selectqgis-ltr.
FAQ
Troubleshooting questions
QGIS works but is very slow on Windows
See the QGIS slow performance guide. Quick wins: enable render caching, use SSD storage, build spatial indexes on large layers, and reproject data to your working CRS to avoid on-the-fly reprojection.
QGIS crashes when loading a specific layer
The layer file may be corrupt or in an unsupported format. Try: (1) open the layer in a fresh QGIS project, (2) try a different file format (convert SHP to GeoPackage), (3) check the QGIS error log (View → Log Messages). See QGIS crash guide.