The procedure entry point_except_handler4_common could not be located in the dynamic link library msvcrt.dll
This error is caused by a 3rd party utility or applcation overwriting your msvcrt.dll with a different version.
For example if a windows vista version of this DLL was copied on to a windows XP computer you would probably get this error.
A solution to this problem can be found in Microsofts KB http://support.microsoft.com/kb/324762
[Edit] Charles Koontz solution seems to be helping a lot of people as well.
Charles Koontz If the problem appears after installing or upgrading the Mozilla Firefox browser to version 3.5 (or later) operating under Windows XP, the problem is due to a Windows Vista driver (dwmapi.dll) installed in a Windows XP OS at folder system32 by mistake (faulty install program). Delete that dwmapi.dll file to fix the problem. Thanks to users of the Mozilla Firefox support site.