How can I know what damage viruses Did to my computer?

 ???

Without knowing what the virus was we cannot give an answer

Given his other post, it appears that the OP is asking general questions rather than specifics.

@ Lir2012
Using the information given in a detection, file name, location and malware name you could could use your friend google.

However, if you don’t know what it is that infected you you wouldn’t know what damage might be done as you can’t search for the information.