No need to uninstall QGIS.
All QGIS settings are stored in the registry at
If you look for the key
HKEY_CURRENT_USER\Software\QuantumGIS\QGIS\locale you will see it has a
userLocale setting. Make sure that
overrideFlag is set to false.
You can also start QGIS with a command line arg to tell it which language to use:
qgis.bat --lang en_US
Once you are in QGIS head to Settings -> Options -> Locale and make sure
override system locale is unticked and that the additional info section gives you the correct language, mine is en_AU for example.