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How do you know they are important other than the file name ?

Some malware use the names of common system files (usually located in different folders) to make the user think as you have that they are important, to stop you dealing with them.

The ones you mention, if they were in fact system files you would notice that rundll32.dll was missing as many other processes require it the same is true of taskmgr.exe you wouldn’t be able to open the Task Manager (though some malware disables this).

So don’t restore anything without confirmation.
What is the malware name, the infected file name and where was it found e.g. (malware name, C:\windows\system32\infected-file-name.xxx) ?
Check the avast! Log Viewer (right click the avast ‘a’ icon), Warning section, this contains information on all avast detections.