I read this question online recently:
I am upgrading from Windows Vista to Windows 7. How do I back up my Yahoo! emails and do I need to do it?
If you have a free Yahoo! email account then there is no need to back up your emails because the emails are stored on Yahoo!’s servers and not your computer.
If you have a paid Yahoo! email account, called Yahoo! Mail Plus, which costs $19.99 per year, then it is possible that you could be downloading email messages to your computer with a program such as Outlook, Outlook Express, Windows Mail, Eudora, or some other program or email client.
Generally, when you are upgrading from Windows Vista to Windows 7 there is no need to erase your hard drive. When the hard drive is erased it is usually referred to as a “clean install”. While it’s always good idea to do a backup before an upgrade, it isn’t necessary. When upgrading from XP to Windows 7 the hard drive must be erased, so this is a case where it would be a good idea to have a backup of some sort.
Still, if you want to take the precaution of doing a backup, go to a computer store and get yourself an external USB drive such as a MyPassport from Western Digital and use the Windows Easy Transfer wizard to copy important files to the external drive. For more info on Windows Easy Transfer go here: http://www.microsoft.com/windows/windows-vista/features/easy-transfer.aspx