Can I replace my old hard drive with a formatted one from my old windows laptop My macs gpu failed and my hard drive decided to go with it, apple replaced the logic board for free but told me to buy a new hard drive. I found a working 80gb hard drive in an old laptop of mine and was wondering if i would be able to replace it without causing any issues (both the same brand).
My own experience is with PCs, but most of this advice should be helpful.

When replacing a hard drive, there are a lot of considerations to consider. You didn't say, so I'm assuming your Mac is not a laptop. If so, you need to examine if the placement of connectors is in the same location. Size is incredibly important for laptops, and most do NOT have a lot of room to play around. Check YouTube for how to open and install it properly. Don't go with the first one - check a couple out before you open up your Mac. And pay attention to safety tips.

You do say that the hard drives are the same brand. The next thing, though is to check the models. If there is a lot of time between the two HDs, the Mac may or may not be able to communicate with the controller board on the HD. 

Most HD manufacturers list what they are compatible with. Like with Western Digital, the brand, are are several types - Caviars, Raptors, etc. Presumably, though, if the HD from the PC is older, this might not be an issue. 

Hard drives have a power connector, a data cable, and there may be jumpers on it to set. If it is close to the same model, set the jumpers to match. Some motherboards require setting slave / master options on the hard drive. 

If this is your first time opening up a machine, (1) UNPLUG the power cable from computer; and (2) Watch out for static electricity. A handy way to ground yourself is to touch the frame. Remember, AC out of the wall can hurt like the dickens. The power unit converts that to direct current (DC). 

* I will say this to others who read this - working on PCs and Macs are fine, and you proceed at your own risk to your hardware. But only qualified techs should attempt to do anything with monitors; messing with them can be fatal to you. It only takes 1 amp to kill you. *

Recommend not working on a carpeted area, as you can build static as your feet move around. If you do have to work on a carpet, take off your shoes and socks. Touch the frame often, as it doesn't take much to damage the more fragile components. Most modern PCs and Macs are pretty hardy inside, but there are limits. As I recall, Apple/Macs are pretty easy to disconnect parts. So you are lucky there.

Another tip. After disconnecting the power, wait 60 seconds before touching parts or components. Modern computers use high refresh rates instead of steady power flow, and it takes that long for all the power through the system to dissipate. The power to your system will NOT go away if you have it turned off. Since the mid2000's, modern motherboards keep a low charge running through it, for things like remote startups and such. Some computers have NIC lights, and that can be an indicator for you. 

UNPLUG it, and wait 60 seconds before opening up the machine. Touch the frame to dissipate any electric charge you might have built up.

So, in summary, see if the manufacturer lists the hard drive as compatible with Macs (it should be; it might just need to be formatted for the Mac to use it first). Check YouTube - enter model of Mac and hard drive. Someone may have done your exact setup, or one close enough to help.

If the PC is older than the Mac, again, it SHOULD be able to talk, but I recommend checking the mfg list to make sure. No sense taking it out if it's not. Lastly, pay attention to connectors and jumpers. 

I like a good, multi-head screwdriver. NON-magnetic! Magnetic screwdrivers are handy around cars. They are probable death to computer innards. Jeweler screwdrivers can get the real tiny screws...

Anyway, hope this helps. Good luck! And be safe!