The Dell and HP ones will most likely report to you that they are for a Dell / HP machine and will NOT install on a custom PC. The Dell might install but not Activate and you would have to call the toll free number and explain to a MS rep what you had to do..and, they might give you a new Key, so keep that in mind..but most likely the HP will not install at all. You can legally use a plain OEM XP Pro CD that you borrow, as long as you use the Product Key found on the COA on the side of your case. It should work fine- you might also need to use a Service Pack 2 Pro CD if that was what was installed and what goes with the COA Key..
The Product Key on the side of the case just has to match the type of XP Pro CD you use to install. Once you know the information, you can record it somehow for next time and won't have to go through all this