on the Hard disk. A full format will remove it and then you have problems. Won't be able to get into the BIOS, lose all of your drivers etc.
If you want to divorce yourself from Compaq and have a legal Op Sys CD, you can first get all of the drivers from the Compaq disk. Previously you could go to the Compaq site and download the files needed to to access the BIOS to a couple of floppies.
Better go to the Compaq site for your Model nd do some research.
I have a Compaq Presario 8000 desktop with Win xp, Pentium 4 2.0 GHz and 256 MB RAM. How do you re-format the hard drive without using Compaq quick restore CDs?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic