If you dual-boot Vista and Fedora or Ubuntu then Thunderbird email client can use a common email box for both operating systems. If you already have an email account set up on one of the operating systems, such as Vista, then open Thunderbird, select Tools --> Account Settings, then click the Local Folders link on the left side of the screen. On the right side you will see the current location of the email files under Local Directory. Write that down someplace so that you can reference it while booted in Fedora.
Now close Vista and boot to Fedora (or Ubuntu). While in that os, install Thunderbird if it has not already been installed (use Add/Remove Programs to do this), then set up your email account. After that is installed, again open Thunderbird, select Tools --> Account Settings, click the Local Folders tab, then use the Browse button to navigate to the same folder that is used in Vista.
That's all there is to it :) Now you can use all the same email messages in both operating systems. When new mail arrives it will be seen by Thunderbird in both operating systems.