I've been trying to install Ubuntu 15.02 in my laptop alongside Windows 7.
When I reach the 4th step of the installation,i.e, the installation type, ther isn't an option for installing Ubuntu alongside Windows 7. When I try choosing the option 'something else' and install it in other partition, an error pops up saying 'no root file system is defined. Please correct it from the partitioning menu'. Please help.
Start by creating an empty partition on your hard disk. The empty partition needs not be formatted to any file system. Once you successfully do that, repeat the same steps you were doing again. I hope your linux will be able to see that you have some free space where it can sit on then it should be able to skip that level.
If you are just trying out you could start out with virtualbox or vmware and install your OS linux there then only when you are sure of what you are doing should you go ahead and attempt to install linux alongside your Windows 7. That way you won't mess up your existing windows installation by accidentally wiping it out while installing linux on your workstation.