You're probably trying to open a binary, not ASCII, file. ASCII is the usual file format for text files such as reports. You will not be able to read this file with Monarch.
What program created the .dat file? Regardless of the program, you'll need to find a way within that program to properly export ASCII text files. Sometimes the only way is to print to a generic printer and elect to output to a file.
.dat files can come from just about anywhere, but there may be clues to be had in the first few lines of the scrambled text you can see. Can you post the first 5-10 lines from Notepad up here in between and tags (without the spaces?
Do you know anything else about what the data is meant to contain?