You can click here for free software that will play most blu ray’s. however, it does not support most of the interactive menus. If you are looking for full support of the interactive menus you will want to take a look at Power DVD from Cyberlink.
If you purchased the Computer with the BD-ROM drive, it should have come with software such as Power DVD for playing Blu Ray media. If the drive was purchased separately most of the time the vendor offers free software so you may want to check the vendor page for your model device to see if there is a free download available.
If you let us know your make / model of your pc we may be able to track down the software for you.