Question: We've got an Asus X501A laptop (dec 2012) which has this UEFI thing. If I boot from an USB it only offers me to wipe the entire hard disc and install the new distro, or manual partitioning which I can't get my head around due to this UEFI thing.
-- Note that this question has nothing to do with that other operating system named after openings in walls. Not interested in that.

Answer: 
1. Get a USB flash drive of at least 1GB capacity
2. [Create a bootable Ubuntu USB drive](http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-windows) by writing a [new Ubuntu image](http://www.ubuntu.com/download/desktop) on it.
3. Boot Ubuntu using this new flashdrive.
4. Install and run [the `boot-repair` tool](https://help.ubuntu.com/community/RecoveringUbuntuAfterInstallingWindows).
If you can boot from USB, your answer is [there](http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-windows)

If your machine has room for more than one HD, and you can borrow one for some time, you can set up the temp HD as primary, install Ubuntu on it and fix the [MBR](http://en.wikipedia.org/wiki/Master_boot_record) from there. Hello screwdriver, hello headheache, but in the end it will be fine.