I've faced with some situation when my Windows 7 crushed and to investigate about it, I've to find memory.dmp file.
But the problem is sometimes it doesn't appears. In order to fix it follow these instructions:-
Click Start > Right-click on Computer > Properties > Advanced System Settings > Startup and Recovery Settings. Take
a look that "Write debugging information" is not set to None. And Dump file location is
default "%SystemRoot%\MEMORY.DMP".
If all things are fine than most probably your free hard drive space is less than25GB. And so Windows will not keep a crash dump file during crashes. But again, you can you can change this settings. In order to do these follow these instructions:-
Start > Run > Type regedit > Okay
Navigate to:-
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Contro l\CrashControl
Create a new DWORD value called AlwaysKeepMemoryDump and set its value to 1.
Your crash dump file will always be preserved in future crashes.