Which version of BearShare?
Which operating system are you using, Vista?
Right-click the program & choose Run as Administrator. In Properties depending on which OS you are using.

An alternative is to uninstall it, then reinstall. Right-click installer & choose Run as Administrator and set compatibility mode of all the programs to Windows XP, SP2.

BearShare on my Windows 7 opens fine. File_Girl did create a separate 5.1 beta installer for Windows 7, but I am not sure if that is the one I have.