I use notepad.exe on an XP SP3 machine. I've lately noticed that the paper size is defaulting to A4, and I can't seem to switch it back to Letter. I've changed the setting in the Page Setup options, ...

The default document size in Microsoft Word is 8.5-by-11 inches -- regular letter paper. However, the paper size for any document can be changed, whether it's an existing document or a new document.

The common letter-sized paper used in Europe and in most locations outside of North America. At 8.3 x 11.7" (210x297mm), A4 paper is slightly narrower and a little longer than "A" size 8.5 x 11" paper ...