Windows Notepad is not the most usefule text editor when working with files over 1 MB.
TextPad works fine with files up to about 2 GB, but opening larger files, e.g. huge SQL dumps of databases, ends up in an error message saying the HDD is full.
A useful free Windows text editor I have found for this situation yet is PilotEdit Lite. It opens files between 7 and 8 GB without problems (haven’t tried larger files yet). Opening the file takes some minutes first, but then works fine.
With Free Hex Editor Neo, you can open large files faster, but takes a bit more time when searching for text in the file.