14 DAY TRIAL // Ubuntu and Windows operating systems can coexist on the same PCs, but if you plan to install a Windows OS, 8 for example, when you already have Ubuntu 14.04 LTS on your hard drive, you will have a bad time.
It's not usually a problem to install Windows when you already have Ubuntu, even if Windows will delete your GRUB and leave you unable to boot your other OS.
Normally, you would install in a Live CD environment of Ubuntu a small tool called Boot Repair. The developer provides a PPA and it's easy to get it, unless you have Ubuntu 14.04.
Boot Repair is not yet available for the Trusty Tahr branch and it is unknown when it will become available. It will probably be before the official launch of Ubuntu 14.04 LTS in April, but right now you need to be careful with dual booting between the two OSes.