A fresh installation is always safer than an in-situ upgrade.
First make sure you do backups of your valuable data.
If you can boot 13.10 or 14.04 from a live CD or USB drive, select
Try Ubuntu. Do not install it.
Open the file manager and then copy and paste the contents of your Ubuntu home directory to an external hard drive.
You should also backup backup your Windows documents, photos etc to the same drive.
Once all your documents etc are safely backed up, you can install 14.04 preferably using the Reinstall option, or alternatively choosing the Erase the existing 14.04 (or 13.10) and Reinstall option.
This should help you:
http://www.howtogeek.com/196740/how-...-it-wont-boot/