make sure
1. no usb devices are inserted ie usb memory stick, external cd or dvd ( optical drive ) , no external hard drives are attached
2. check in bios to ensure that you have your main hard drive selected as the bootable device
3. if the above is true then your Master Boot Record or Boot.ini file is knackered which you can repair but you will need your installation operating system disc to do so that or create a bart pe disc along with relevant plugins to repair the MBR and edit the boot.ini file
Post back if you get stuck
