Windows Live Mail is the 'new' Outlook Express effectively.
It's actually not bad in the Win 7 / 2011 version. It's missing a load of stuff you'd find in the full Outlook and it does require a Windows Live / Hotmail account to work, but you don't have to use Hotmail for your mail once you've got it running.
For someone less computer literate it's pretty simple and intuitive, plus nice looking in Windows 7.
Thunderbird is a little more complex but does more. I find it a bit clunky and slow though and the UI is dated now.
Outlook is the full blown option, but it's way over the top for what most people need, and is very expensive with Office (compared to just Word, Excel and Powerpoint).
Or if you just want to use Google Mail, could just use GMail client in the browser. Chrome even has an offline app in the browser for GMail.
P.S. PST files from Outlook won't load in pretty much anything but Outlook. Proprietary format and tools that read the files cost money. Thunderbird can read them, but only if you have Outlook installed on the same computer.
However, I can highly recommend MailStore (
www.mailstore.com) which is a mail archive product that transfers mail between different mail apps pretty well. The home version won't read PST files, but it will archive from Outlook on a PC that has it installed, then you can restore into another mail app, and you can do one time with a free trial. The pro version reads PST files also, but costs more money.