That is a very broad question.
The 'damage' that a virus can cause to a system can vary greatly.
This can range from annoying changes to even destroying the Operating System to the extent that the only cure is to wipe the Hard Drive and completely reinstall everything from scratch.
Presumably you have a reason for asking, is it curiosity or have you issues?
It depends how important data you have on your PC too... If hacker lock your HDD and wants from you to pay some amount of money so you can get your data back, well that could be bad thing...
So you should always have backup in mind, because if something like this happens you can always wipe your existing OS and do the clean installation and not to be worried if some of your data would be lost...
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