Question about how Antivirus software can repair severe virus infestation

[jimdagys]

I know that a virus can attack an exe file and make the exe unusable. I also know that a virus can mess around with the Windows registry.
My question is: Since there are many exe's in Windows operation system, I don't understand how Antivirus software can deal with Windows damaged exe's or Windows registry that has been already altered by virus.

[edDV]

Once a serious virus has done its deed, it is very difficult to fix it. One would need to understand the details of the virus to reverse the infection. Best to reformat and start over.
http://en.wikipedia.org/wiki/Antivirus_software

[jagabo]

Take the following sentence:

"The cow jumped over the moon."

Replace the word cow with cat:

"The cat jumped over the moon."

Now replace the word cat with cow:

"The cow jumped over the moon."



Or take the following sentence:

"The cow jumped over the moon."

Insert "I feel sick. " at the start:

"I feel sick. The cow jumped over the moon."

Remove "I feel sick. " from the start:

"The cow jumped over the moon."

[hech54]

If you are talking about a recent incident with Avast....ignore it....it's a burp.