"Comments" are a JPEG header item. There's a 0xFFFE comment marker in the header, followed by a 2-byte length field. Comments are unformatted free text.
The confusion comes from Windows *also* calling textual photo descriptions "comments", which are stored in the JPEG header but different from JPEG comments.
I've found there are significant problems with using Windows 7 Explorer (also likely Vista or XP) to enter JPEG comments.
(1) The JPEG comments read/written by Explorer in Windows XP, Vista or 7 are different from the normal JPEG comments typically accessed by other utilities. Comments written by Windows are typically not readable by other utilities, and vice-versa.
The confusion stems from both being called "comments", and Windows displaying those alongside other JPEG header fields -- which implies they are JPEG comments. In fact they are Windows comments, stored in the JPEG header but distinct from JPEG comments.
(2) Windows "JPEG" comments are not searchable by Windows 7 search. Since they aren't typically accessible by 3rd-party utilities, this effectively means you can't search on photo comments entered by Windows Explorer. It's really a deplorable situation.
There is one command-line utility ExifTool that can read & update both jpeg and "XP" comments entered by Windows Explorer: ExifTool by Phil Harvey
Provided you know all this, the solution is simple: just don't use Windows Explorer to add JPEG comments, and don't use Windows Search -- use a 3rd party utility to enter/search the JPEG comments. You also must always remember those comments won't show up in Explorer -- the comments field will just be blank from the Windows standpoint.