Why can I tamper the file?

Why can I tamper the file?

To demonstrate the usefulness of signatures and digital fingerprints I loaded mspaint into a hex editor and altered one character from P to X (the character was located in a string). When I later on launched mspaint I was expecting Windows (xp) to pop up a dialog to warn me and say that the file had been tampered. But no such dialog popped up! Why?