As I suspected, you have been infected with a Bitcoin miner. This is a kind of malware that uses your computer's processing power to generate Bitcoins for the attacker. Because this is a very resource intensive process, it can cause severe lag and unresponsiveness.
Source: Cybercriminals Unleash Bitcoin-Mining Malware
This software could have gotten onto your computer in a couple different ways: a drive-by download or as a result of an attacker having remote access to your computer. If it is the later then there is a very high likelihood that your computer's security has been tampered with to allow the attacker remote access in the future which can be done in ways that are undetectable to us. This would allow them to reinfect your computer easily. Unfortunately, there is no way for us to know which one it was.
We have a couple options for how to proceed:
- Remove the Bitcoin miner and be done with it. This has the highest risk of reinfection and if you do any kind of online banking or shopping the risk that your banking or credit card information gets stolen is too high for me to be comfortable suggesting this. I strongly recommend against this option.
- Perform a Reset with the Keep my files option (also called a System Refresh). This will reset Windows and remove any installed programs but will keep any personal files, documents, PDFs, etc. This will likely remove any hidden malware or backdoors. Because some files are not deleted this is still some risk that malware could slip by but I think in this case the risk is small enough to be acceptable. This is the option that I would recommend.
- Backup your files and perform a Reset with the Remove everything option (also called System Reset). Because you would need to back up files manually this option practically guarantees a clean computer. It is the most time consuming option but also the most secure. In this case, I believe that it is probably overkill.
Please let me know how you would like to proceed. There is 3 days before this topic gets closed so you have some time to think it through. If you need additional time just reply within that time period and let me know.