Depending on the Make and Model of your machine you may need to hit Delete ( which is usually the industry standard ) while it boots up. If you start hitting Delete several times after you power on you should be able to access the bios...
Once you are there you need to check for boot device.
You need to see what order the boot device is set to.
Usually people boot to
Floppy, then Cd-ROM and last is the Hard drive.