I am a computer engineer and I recently had to reinstall Windows XP on a client machine, as it was struck with a nasty piece of malware, AntiVirus XP.
Before the reinstall, he had Outlook 2003 on his system. I saved his Outlook configuration file, his address book and email. Now he hasn't got access to Outlook 2003, he wants to try Thunderbird. How do I transfer all his Outlook emails, address book, and settings to Thunderbird?

These links should help you out. Importing Outlook Express Emails Into Thunderbird And Evolution | HowtoForge - Linux Howtos and Tutorials
Importing Contacts in Mozilla Thunderbird - import from Microsoft Outlook, Outlook Express, Qualcomm Eudora or text files (like LDIF for LDAP directories)
Outlook to Thunderbird and Back Again - Import and Export - Windows | Blog on a Stick