Sorry for the late reply, but the scan took 13 hours for some reason. Now after i have done a full threat scan including a root kit scan, malwarebytes has not found anything and said that my pc is completely clean. I also used the chkdsk command and it came up fine as well. I would like to clarify that the pc is not at 100% usage always, but i see that happening often. I cannot pinpoint what times this happens or what might trigger it. It affects performance when sometimes the os takes time to respond like the taskbar or some apps "waiting to respond". I have also run a chkdsk /f /r and it did not give any errors with the drive.
When this kind of stuff happens it can get to the point of wanting to swap things. But since all you did was bang the desktop and it was originally working fine, I would suspect one of two things. I'd ask myself, did I crack a circuit board and did I really need to replace my hard drive. I'm hoping you have your old drive as you might put it back in and see if the computer still acts the same way. Also, think hardware and forget about software for a bit. Let's take a flash light and a magnifier and examine the circuit boards carefully. Especially around any area where two heavy items interface. Be careful in there, turn off the power, unplug the computer and hold in the power button 10 seconds to facilitate capacitor discharge. Also beware there can easily be high voltage stored in the capacitors inside of the power supply protective case even with the power off and it's unplugged. Don't touch those parts without using a non conductive tool. When you look around always keep one hand on the computers metal to prevent static. Also get some cardboard to set the computer on. It helps to wipe it with a damp cloth first just to get some moisture in the cardboard. It's an old anti static thing. Your probably going to have to pull the cards out to examine it. So get plenty of cardboard to lay your cards on. Pull the panels so you can really see that mother board on either side. Look around the heat sink area because it's pretty heavy with the fan on it. Hopefully the heat sink area where the screws are didn't crack the circuit board. Same goes for heavy graphic cards and so on. Also examine the capacitors on the mother board. The larger ones are vented and if they have problems they can swell on the top. Old boards subjected to a lot of heat can dry out the electrolyte inside the caps and cause unstable computers but you can't see that. But look carefully for any signs of discoloration. This is common when a hot running part is right next to it. You may see this more inside of your power supply than the computer itself. However given the timing and all was fine before you jarred it, it's probably cracked or has a loose connection somewhere. It doesn't necessarily mean a circuit board has physically cracked but a heavy or hot running part with a prior partial solder joint may have broken apart. This is the most likely problem, so lightly touch each large part and gently move it around. Most parts should not budge but if something does where it is soldered this may be the cause of your intermittent problem. It doesn't take much to move a leg that is suppose to be soldered. Most likely you won't find anything as most modern boards solder things well but its still worth looking. You may have a dirty circuit board and want to clean it. This is where it gets tricky, you have to use Anti Static ESD Brushes that don't build up static and don't even think about using the vacuum cleaner.
For starters I'd try swapping the hard drive with the old one as it may actually be ok. If it is then before spending any hard earned money take a serious good look at it. You might not be able to repair a cracked pcb but at least you'll know what to do.
If you don't find anything physically remember that you are effectively cleaning the contacts when you plug things in and out. I normally use a chemical called De-Oxite - G5. It comes as a spray but I spray an anti static brush with it and brush the contacts clean before reassembly.
So even though it sounds like a lot, it's something that couldn't hurt because at least you have cleaned your connections. Note that de-oxite may need to dry 15 to 30 minutes before power as reapplied. It is non conductive but it can act as an electrolyte and cause instability. So if you do use it, make sure you get any excess out. You can blow excess out with canned air but it can blow right back in your face, not good.
So just think of it as giving your computer a good cleaning. I hope it's not full of cigarette smoke. Believe it or not, that can cause problems and there are special chemicals to remove it.
I'm thinking you may find loose connectors as you disconnect and re-seat them. Especially those large 8 or so pin power supply connections. It's very possible one of those is loose and causing intermittent problems. Most definitely check and clean those contacts first. It may be the very heart and soul of your problem. Poor power connections cause all kinds of problems.
That's a head full huh. Good luck and go easy, one step at a time. Don't loose your cool, somethings gotta give.
Almost forgot to ask, was the hard drive you got brand new? 13 minutes for a full scan could be normal if the drive has a lot on it. But if you bought a used one, it may have been someone elses junk.
Also you need to monitor the power supplies with software under heavy use and make sure the mother board is properly powered. A slight under voltage to the micro or memory can cause problems. Do that first, I think we missed that software step even though it is hardware power your observing. There are lots of voltage monitors, I have an Asus mother board and it has it own software for that. Not sure what yours has, but definately monitor your voltages before doing a visual check. You may have power supply issues.