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 as long as one of the drives is a WD.
3. Remove the HP drive and install it in the Compaq. You can usually jsut grab the power and signal cable from the CDROM drive. No need to physically mount the drive -- the power connector has ground you can just let it hang there. (yes, i do this, it's probably bad, but its easy). at 40GB these are old IDE drives. Make sure the master/slave/CS jumper is set correctly.
4. Run the cloning software. For 40GB shouldn't take all that long but it's not quick.
5. Remove the old drive from the compaq, replace with the HP drive. Again make sure jumpers are right.
You should be good to go.