If you need to re-install a 'hidden' Windows component, eg. outlook express or messenger, there is an easy way to do this... [but you will have to BE CAREFUL!]

Open the INF folder in your windows folder [C:\WINDOWS\INF]
Find the *.inf files [view>arrange icons by>type]
Find the application title you need in the filename [for example messenger is msmsgs.inf] If you are unsure of the installer, RIGHT CLICK on the file, then select open, at the top of the file should say what application it is:

; Setup INF file for Messenger

When you have found the file the right click on it then click install.
Tip: To reinstall Microsoft Outlook Express, Right click and select install on MSOE50.inf