The CD I am using is from a Dell desktop
Is it for THIS model of Dell laptop?
If not you, then you have drivers on this cd that will not work on the current system.
Therefor your installation will not be successful.
This is consistent with your 0x50 error.
You will need the cd that Dell shipped with the system, or else a full version of Windows.
You may or may not be able to find retail packages still available.
Some sources seem to have them priced higher than Win7.
As for your hard drive... if you know the maker, they will have a utility for their drives.
This utility often will run outside of windows. Again, you would burn a disk for this.
If your hard-drive fails either the short or long test it will need replacement.
As for Memtest... yes... burn the iso to cd. It runs in it own environment.
Please follow this guide...
https://www.techspot.com/vb/topic62524.html
Please note that (unless you start getting errors reported) you will need to let this run a long time.
A good way of doing this is to let it run over night. In each 'pass" there are a series of eight memory tests that are run, each more complicated than the previous. You need to let it run a minimum of 7 passes, unless you start getting errors. If you have errors reported, you may want to re-run the test with only one stick of ram installed, checked each stick until you isolate those that are faulty. That way you can limit your replacement to those that actually need it.
Let us know the results. If your memory comes up without errors, then we can check other things.
HTH.