The event viewer is a windows application which records information about software running on your rig (amongst other things). You can find it by right-clicking the windows icon in the bottom left of your desktop, select 'Computer Management, expand 'Event Viewer' (click the arrowhead), expand 'Windows Logs' then finally clicking 'Application'.
Once you're in the application log, you need to look for errors occurring at the times when BoS has crashed. If you can't be sure try running the game to the point where it fails and then immediately check the log file. When you click on an error, the log will tell you which application has crashed and will give you an error code (like 0x5776ade0 for instance) - This code sometimes is useful to tell us what's happened. If you can post that error up we can look at things in a bit more detail.