MacBook with Powerpoint issue

If your system is working just fine with only Microsoft PowerPoint having issues I don’t see this as a deep hardware issue, more likely you got infected with some malware which is messing you up when you run the infected app (PowerPoint).

Even still you have an older mac system which can have issues with the HD SATA cable and if you are still using a HDD (spinning disk) Vs running with a SSD (solid state) the drive maybe badly fragmented and have errors.

So… I would first replace the HD SATA cable following this guide: MacBook Pro 13" Unibody Early 2011 Hard Drive Cable Replacement. Being careful not to damage the cable in the process. You don’t want to crease the cable as that will damage it. You also want to put down a stip of electricians tape on the uppercase where the cable crosses over Your Hard Drive Cable Is A Ticking Time Bomb

If you have a HDD then you’ll want to run Disk Utilities and run First Aid to checkout the drive for corruptions if you can’t clear the errors you should look at getting a new drive, if you can clear the errors you should run a defragmenter app on your drive ProSoft - Drive Genius 5 being one of the better tools.

If you are thinking of upgrading your drive or replacing it you should look at getting a SSD. Here’s iFixit’s kit MacBook and MacBook Pro (Non-Retina) SSD Upgrade Kit

Now with a better hardware setup, you might want to re-install your OS and app’s by first making a full backup of your drive using TimeMachine and setting up a bootable OS installer using this OS installer How to upgrade to OS X El Capitan jump down to Step 4 and click on the Blue URL link: ‘Download OS X El Capitan’ then follow this guide to setup a USB thumb drive as an OS installer: How to make a bootable OS X 10.11 El Capitan installer drive Make sure you format the USB thumb drive with GUID with a Journaled file system as the exFAT format won’t work properly.

As to why you want to go through all of this is Apple has a certificate system in the installer which is likely expired! Here’s more on this aspect If you've got an old macOS install image, it will probably stop working today

Once you’ve setup your bootable OS installer (give it a try) you’ll want to reformat your drive first and then reinstall the macOS. Then I would install a good antivirus/malware add/s to check your TimeMachine backup before you restore it using Migration Assistant. If you can I would install a fresh copy of Office so you know the app is clean of anything.

If you want to you can upgrade your macOS to Sierra I wouldn’t go any higher as High Sierra has issues on older SATA based systems as it will convert your file system from HFS+ to APFS which is very chatty!

Yes! this is a lot of steps, But it will get you going again!