Short answer: Yes, Administrators can view your private messages. Mods cannot.
Having said that.... We do not go snooping around in your messages. I can only think of one time, about 5 years ago where I went in and checked someone's messages... and this was because one user allegedly had threatened to kill another member.. and I had to find out if this was true.
Personally, I consider going through somebody's private messages rude and an invasion of privacy. So only under certain -rare- circumstances would I do so.
This is something to keep in mind at all the internet message boards that you visit. most all forum software allows the administrators to access your private messages.
Be careful with your sensitive information.
(BTW, neither admins or moderators can see your password.. we can reset one for you... but we can't see 'em... so rest easy there.)