Start the PC.
Press Del a few times.
You will enter the BIOS.
Goto Boot Settings, or advanced Boot Options.
Change the order of the Boot Device to CD/DVD Rom First, as Primary Boot Device.
Change the Secondary Boot Device to the HDD.
Disable the rest of the Boot devices. And also disable Boot From Other Device.
Press F10. Save and exit BIOS.
Remember to have a Bootable Windows or other OS disc in the CD/DVD drive.
If it asks you to boot from CD/DVD press yes or a key within the given time.
Check and see if it Boots.
After you choose the Installation Partitions and the copying and loading is completed the PC will ask for a reboot, press yes and as the rig goes into post start pressing Del. Change the Boot order in the BIOS back to HDD first primary boot device.
Disable all the rest. Save Exit and reboot.
The installation will then continue. Just remember to leave the OS CD/DVD in the CD/DVD Drive, 'cos windows will need it again to copy a few files.