This message seems harmless, I don't think you have to worry about having been hacked. It can appear even on a freshly installed operating system (including Windows 8.1) when other computers are on the local network.
One of the network functions in older Microsoft networks is called a "Master browser", and there is typically only one of them on a LAN. This message says that another computer is claiming the role of master browser, and that the computer you see the event message on is stopping its master browser service.
IMHO this event shouldn't be classified as an error. It looks like there is a typo in the event source name, the word "bower" should probably read "browser". In older versions of windows, the the event source name was "mrxsmb".
For more detail, see http://support.microsoft.com/kb/188001/en-us