How can I configure Outlook to be the default mail client?

Follow the below steps to make Microsoft Outlook as Default Mail Client.

Make Outlook Your Default Email Program in Windows 98, 2000, and XP

1. Start Internet Explorer.
2. Select Tools Internet Options from the menu.
3. Go to the Programs tab.
4. Make sure Microsoft Office Outlook or Microsoft Outlook is selected under E-mail.
5. Click OK.

If the above steps doesn't works try the below.

1. Start the registry editor (regedit.exe)
2. Move to HKEY_LOCAL_MACHINE\SOFTWARE\Clients\Mail
3. Double click the (Default) value and set to "Microsoft Outlook" (don't type the quotes)
4. Move to HKEY_CLASSES_ROOT\mailto
5. Double click the (Default) value and set to "URL:MailTo Protocol" (don't type the quotes)
6. Double click the EditFlags value and set to 02 00 00 00
7. Double click the URL Protocol value and clear
8. Move to the DefaultIcon folder (HKEY_CLASSES_ROOT\mailto\DefaultIcon)
9. Double click the (Default) value and set to"C:\Program Files\Microsoft Office\Office\outllib.dll",-12
10. Move to HKEY_CLASSES_ROOT\mailto\shell\open\command
11. Doucle click (Default) and change to"C:\Program Files\Microsoft Office\Office\outlook.exe" -c IPM.Note /m "%1"Alternate values (such as Lotus Notes) are:"C:\Program Files\notes\notes.exe"=C:\WINNT\notes.ini %1
12. Reboot the computer

Note: Verify as well the registry key HKEY_CURRENT_USER\SOFTWARE\Clients\Mail should have the default value as Microsoft Outlook

Where Office determines the release of Outlook as follows:
Office 10 - Outlook 2002
Office 11 - Outlook 2003
Office 12 - Outlook 2007