You have a few options:
1. If you are "booting" from a DVD/CD, you may have to go into the BIOS and enabled the DVD/CD drive in the "boot order". Most PCs boot from removable drives first, then they goto the fixed drive, usually C: It is possible that you will need to enable your DVD/CD drive in the BIOS and then re-boot.
2. If you are re-loading, you may be able to run the Windows DVD/CD from within your current installation of Windows. If this is possible, when prompted, choose a "Clean" installation.
3. Alternatively, many big name PCs have a "recovery" partition on the existing hard drive - have you checked your Start menu for items like System Recovery?
Rgs
--Craig