I had a harddrive that was running as a slave. I installed windows 7 to it and it booted fine, so i took it out and put it into another computer but when I try to boot it as primary there it gives me a screen that says something like - new hardware is detected with option to boot as normal or boot in another mode (im guessing it tries to find the hardware difference).
This is the only HD on that system..is this a normal thing? its an HP originally, but im mixing new parts in.
Please let me know if theres something that should be done in transferring hard drives to a new system with OS already installed.
Yes, this is expected. The drive has drivers for the hardware on your old system, not for the hardware that it finds when it boots.
There are two main solutions. Make a backup of your drive first, just in case. The first is to boot the new machine from the Win7 distribution DVD and choose a Repair Install. This will install the missing drivers. Be sure to set the mobo drive controllers to AHCI mode first and to do a Windows Update afterward to get drivers that have changed since the DVD was cut.
The second is a backup-restore utility with an option to restore to dissimilar software, like EASEUS To-Do backup.
If you can boot to USB, make a bootable thumb drive with your Windows 7 disc copied over (4GB+ drive). Either that or get an IDE or SATA DVD drive (if supported/capable) or even a USB slot loading DVD drive (again, if supported). Heck, pull the DVD drive out of you other PC and put it in your "arcade" system temporarily.
Option B does requires that you boot from another device to do the restore. Then again, option A requires that you boot from a DVD drive to do the repair. Listen to TdiT and see if you can boot from an external device. I have EASEUS To-Do booting from a thumb drive (along with a choice of five other live environments. What a tool).
EDIT: The phrase "What a tool" was admiring my boot drive, not describing myself.