How To Reinstall Ubuntu
Reinstalling Ubuntu's system files may sometimes be the quickest way to solve a problem, eg. if an upgrade failed or if your graphic driver is broken.
Few people know it, but since Hardy it's possible to reinstall Ubuntu without losing the content of the /home folder (which contains program settings, internet bookmarks, emails... and all the documents, music, videos that you have put in it). Even if /home is not on a separate partition (which is the case by default if you did not manually separate it). Remark: this tutorial can also be used to upgrade Ubuntu (eg 11.04 -> 12.04 from a 12.04 live-CD).
Here is how to do it simply:
Before all
This operation should not damage your documents but, for security, backup your documents and settings (including /home hidden files) on external disk or DVDs. (eg via CloneZilla) Remark: some special applications settings may be in system folders, eg LAMP, see below in the thread.
Run Ubuntu installer
- run Ubuntu installer
- go until the "Installation type" (or "Allocate disk space") menu
- then choose the right option as described below:
Choose the right option
2 possibilities:
- If an "Upgrade 1X.XX to 1Y.YY" option is available (like in the screenshot below), choose it. 
- Or, if the above option is not available, choose manual partitioning ("Something-else" option), then select Ubuntu system partition, set its mount point as "/" (keep the same format type, the same size, and untick the "Format" checkbox). Also set other partitions (/boot, /home... see DiskSpace) if needed.
Then finish the installation process. (remark: this may take several hours, like a normal install)
After reinstalling
After reinstalling, user accounts must be re-created with same login and password.