I have a emachine t6212 with a working harddrive but bad motherboard. I think the onboard video went. It had squiggley lines for months which slowly got worse now goes black after startup. Its running an upgraded windows 7. Which i no longer have the disc for. I bought a used hp compaq Dx2200 which has windows xp pro on it no disc either. I want to install the old drive into the new computer. The new one has a sata hardrive and the old one is a pata drive. I removed the sata and put the pata in set to master and it recognized it but when it goes to start up it goes to a black screen with _.Never shows windows screen or anything. Yes the drive works in the old one it will go to the home screen but then blacks out afterward.
You should first test the old PATA by booting into the OS on the SATA drive and seeing if the old drive is corrupted.
Even if you are able to transfer the old HDD to the new(er) rig You will have to set the BIOS (or at least have the BIOS recognize the PATA drive) and install an operational OS to the PATA. The set drivers will (99% probability) not work with the new rig. So a fresh install of the OS is necessary.
It may be that all you have to do is install a new OS in the new rig, as the BIOS seems to recognize it. It would suprise the bejesus out of me if the old PATA could use the installed version of Win 7 on the new rig.
The main problem that you have is that you are trying to boot an installation done on one set of hardware using another set of hardware. This rarely works. The installation process chooses drivers that match the motherboard and the controllers on it, and these are used early in the Windows startup process. So when you boot the drive in another machine, it will usually black-screen.
The usual solution is to boot off the installation disc and do a repair install, but you never had full installation discs with the necessary drivers. The discs that you lost / did not have were almost certainly specific to the hardware. Vendors do that these days; the install the OS on the machine and the discs that you get are an image of that installation; they don't contain the alternate drivers at all.
In addition, if the OS on each machine came with the machine, what you are trying to do is against the license agreement. Finally, old pata drives are slower than current drives.
What am I trying to say? You can't get there from here. Your second-best solution is to borrow a Win7 installation disc from a friend who has a licensed copy and boot it, then do a repair install. Your best solution, should you be able to afford the cash, is a new machine, or at least an SATA drive and an OEM full copy of Win7. Sorry to be a wet blanket, but you are not in a good situation.
You will need to re-install for things to work in the new system. Unfortunately MS defines a machine as a mobo and a case, so when you move out of that realm then you are breaking your license agreement. While MS is willing to work with system builders on their home rigs that often morph into completely different machines over time, they are quite sticklers when it comes to pre-built systems like eMachines, Dells, etc.
Win7Home OEM is $100 on newegg. Go with the 64bit version. If you have a domain, or need good 'compatibility mode' support then go with 7Pro, but 7Home is good enough for 99% of home builders.
Almost forgot, your old PATA drive, and the files on it, are probably fine. Install the PATA as a 2nd drive to the SATA (PATA will remain as master on the PATA cable for the jumper settings, change the boot order in Bios), and then when in Windows you can browse the drive and copy the files you need off of it, and perhaps format it as a backup or documents drive.