First - your thread title. Outlook Express will not work in Windows 7 and has been superceded by Windows Live Mail.
Second - to boot your Compaq and format the drive for a new installation you need an XP CD. Turn on the computer and press whatever it takes to go into BIOS or Setup. Look for the Boot Sequence or Boot Order and change it so that the CD ROM is top of the list. Save the changes and exit then when a bootable CD is in that drive, the system will use that boot option first. Part of the install process includes formatting the drive which will wipe out everything on it.