Are you a sysadmin or in the dbm_moniotr role? It is also possible that you have both tools for 2005 and 2008 and maybe the pack for 2000 in 2005. The installation steps and sequential order in which these are installed is touchy. It's possible the sqlmonitor.exe is having issues that you are selecting. Possibly try the sqlmonitor.exe in the 100 and 90 Binn folder and see if you get the same results. You can mannually launch them by opening sqlmonitor.exe-->Go-->database mirroring

I would also search on SP3 or an updates on msdn for any known updates or fixes in 3042 (SP2).