For the question related to installing Ubuntu on a computer with Windows 8 see the answer in Installing Ubuntu on a Pre-Installed UEFI Supported Windows 8 system that should cover everything related to installing Ubuntu on a computer with Windows 8. Note that if you do not suffer from the UEFI issue, you only need to install Ubuntu. The grub will simply work and grab the Windows 8 partition. Tested on way too many computers / laptops.
If you however, did it backwards, with this I mean, you have Ubuntu and installed Windows afterwards. Then you need to follow the answers found in How can I repair grub? (How to get Ubuntu back after installing Windows?)
In any case, since 13.04 came out, I recommend using 13.04 over 12.04. You will have better support for issues like this, more hardware support, more fixes, optimizations and more. For a brief explanation of why I would suggest to go with the 13.04 release please read install 13.04 or stay with 12.04 LTS?