I assume you overwrote your old operating system with XP without doing anything like format, fdisk and deleteing files from your old windows directory , also I assume your old system were installed under "windows " directory, then search xxx.wab where xxx stands as your computer name you specified in your old system under "windows" directory. Once you find it, import it into your outlook express by doing this:
Open your outlook express, go file->import->address book, you shall get a box by which you can specify the xxx.wab you just found, then click open.