In Windows Notepad, you cannot (in general) mix fonts, you can only select one font at a time. However, this statement has to be qualified for recent versions of Windows.
Windows XP and earlier
On Windows XP and before, Notepad could not display characters which were not in the selected Font. Therefore a missing or incorrectly displayed character (typically shown as an empty box) could be caused by:
Using Arial instead of Arial Unicode.
Arial is 778,552 bytes, Arial Unicode is 23,275,812 bytes. The difference is that a huge number of characters are in Arial Unicode but not in Arial. (these sizes are from Vista not XP but the difference will be similar).
Omitting the Byte Order Mark(BOM)
Windows expects Unicode files (UTF-8, UTF-16 LE, etc) to contain a BOM. If not,
Windows then guesses the encoding using a Windows API function (
isTextUnicode) which is well-known to make mistakes, resulting in multiple Latin-1 characters being shown instead of a single correct character.
Windows Vista onwards
Notepad has now adopted the strategy first seen in web-browsers - if the character does not exist in the current font, find a font that does contain that character and, for that character only, use the other font. Therefore if you have different additional fonts on one computer, it can behave differently to other computers (even though the OS is the same).
I don't know the algorithm used but it seems possible that if you have a corrupted or much smaller Arial Unicode that lacks some characters, Notepad may believe the font contains a character it does not. An Arial font of 3,395 KB is not what I would expect on Windows 7. Perhaps installing some application has replaced the default font with one that is faulty in some way?
Additional "bad" font
Alternatively Notepad might search a different, recently added, font for the missing character before looking at Arial Unicode. If this different font claims to contain the character but does not (e.g. incorrect layout tags, Notepad could fail to display the character
On the computer that has the 3,395 KB Arial regular, copy the font file to a safe place, un-install it, then install the 761 KB Arial regular font file copied from the other computer.