I would most definitely install windows first, and the reason for that is:
- This method is recommended by Ubuntu
- It takes very little knowledge and all knowledge needed is easily available here: https://help.ubuntu.com/community/WindowsDualBoot
- You dont have to think about how much space is required or save any partition space for linux. Simply install windows on all of your available space.
- WHEN you start installing Ubuntu simply select "install along side windows", pull the handler to the amount of space you want Ubuntu to have available, and the installer will take care of all the partitioning (Which can be complicated for new users).
- Finaly, IF you install windows AFTER you install linux. Windows will delete your boot record, meaning you would have to reinstall it. Thus making things more complicated then it has to be.
Usefull tips: If you have trouble booting into linux after installing. Have a look in your BIOS and look for safeboot and (and i stress and) se if this in anyway prohibits other operating systems (It will say so). If so disable it.
Good luck to you :)
PS: You might want a little more than 15GB... (You might like it :) :)