The HD on the compaq has some things on it you want
1. copy of windows
2. drivers tuned to the compaq ssytem that are not part of the standard windows install
3. compaq recovery partition.
4. all your brothers data & applications & music, etc.
Suggest you clone the Compaq drive onto the HP drive you you end up with two exact copies of the conpaq drive. Then replace the compaq drive with the HP drive.
Steps to clone the data:
1. Find out the real manufacturer of the drives. Windows will tell you in device manager. Compaq and HP do not make drives, they buy them and don't hide that.
2. Go to the drive makers web site and download cloning software. Fro example, western digital has a copy of acronis true image you can use on WD drives...