If you have a problem with Mumble and wish to help the developers in fixing it, it usually helps to give Mumble developers a debug log of what Mumble was doing before it crashed/had a bug. For this purpose of debug logging, there is WinDbg.
Now install with all the default options but click "Complete" when it comes to the installation type.
If you are on Windows Vista type "windbg" into the start menu search box and press enter. If you are on Windows XP click Start -> All Programs -> Debugging Tools for Windows (x86) -> WinDbg.
Now WinDbg should be running. Press Ctrl+E and in the window that pops up navigate to "mumble.exe", click it, then click "Open".
Mumble should now start to load. Whenever you see a line containing "int 3" just keep pressing F5 until Mumble completely starts and is functional (providing that it actually starts without crashing).
When you come to the bug or error that you saw in Mumble and the log stops displaying any more data, copy and paste the log into a pastebin and give the link to a developer. The developer can hopefully look over the log and get clues to what the bug or error might be.