You need to run the samba server to do that. Also open the ports that are needed as well. When you install and configure samba using Ubuntu's wizard, the latter part may be done for you as well.
What filesystem is used on the external drive you want to share? If it is ntfs, it may make more sense having it plugged into your wife's Vista computer. Linux can't fully repair or defrag an ntfs filesystem.
This book has sample Samba configurations you might want to look at. It will take you step by step in configuring samba and even using the correct permissions on a directory you want to share.