For general advice see Troubleshooting issues with iTunes for Windows updates.
The steps in the second box are a guide to removing everything related to iTunes and then rebuilding it which is often a good starting point unless the symptoms indicate a more specific approach. Review the other boxes and the list of support documents further down page in case one of them applies.
Your library should be unaffected by these steps but there is backup and recovery advice elsewhere in the user tip.
If you've already tried a complete uninstall and reinstall try opening iTunes in safe mode (hold down CTRL+SHIFT) as you start iTunes, then going to Edit > Preferences > Store and turning off Show iTunes in the Cloud purchases. You may find iTunes will now start normally.