Many people swear by Outlook, which is part of Microsoft Office. It combines email with calendering and other personal information and I understand it is quite handy. I know nothing about Outlook personally — how secure it is or anything else. Nor do I intend to find out. :-)
Thunderbird is my email client. While I use Gmail for my main account, I use Thunderbird for some special purpose accounts. It's the client that I recommend.
Windows Live Mail is Microsoft's superior replacement for the outmoded Outlook Express and Windows Mail (comes with Vista). Windows Live Mail has some good safety and security features.
Outlook Express isn't such a bad email client if you "lock it down" to avoid the many hazards of email.
Don't forget to backup your email with a good backup program.
For example, in Thunderbird, after you've created your account(s) do the following: Click Tools > Account Settings > click Server Settings for each account (you may have more than one set up) > click "Leave messages on server".