I'm running V5.5.7.  What a workhorse!

I'd like to modify the email messages that are sent out when a user registers and when people post replies to subscribed discussions, but I cannot find what is generating them. 

In particular, the subject line just says "Notification".  When a new users registers, for example, I'd like more info, like "New User Notification: <userid>" or something like that.  Just so I don't have to open the email to see exactly what it is.

Can anybody suggest what's needed here?

TIA!
-David