In Outlook Express/Windows Mail:
Ensure that the Watch/Ignore column is visible in the message list panel (it
has a reading glasses icon); if it's not, right-click in the column headings
area and select it in the list of Columns. Then, for any message you want
to watch, click in the Watch/Ignore column to show the reading glasses icon;
messages in that thread will be highlighted in red, and new messages will be
bold red. You can click again to show the Ignore symbol (circle with
diagonal stroke); you can combine this with a View setting to hide read or
ignored messages - useful to prevent irrelevant message threads appearing.
Click again in the column to clear the watch/ignore state.
It's an excellent, easy-to-use method, which I use all the time.
HTH,
Rob