I don't know the size of the "c:\1.avi" file but the size of the buffer you give to Windows (256M!) is probably big enough to hold the file. So windows decides to read the whole file and put it in the buffer the way it likes. You don't say to windows "I want async", you say "I know how to handle async".

Just change the buffer size say 1024, and your program will behave exactly the same, but read only 1024 bytes (and return ERROR_IO_PENDING as well).

In general, you do asynchronous because you want to do something else during the operation. Look at the sample here: Testing for the End of a File, as it demonstrate an async ReadFile. If you change the sample's buffer and set it to a big value, it should behave exactly like yours.

PS: I suggest you don't rely on time samples to check things, use return codes and events