Vlk said this in the past:

avast was compiled using MS Visual Studio.NET, that is, for the case of avast, these files would be called msvcr70.dll and msvcp70.dll.

Igor said this in the past:

That's a common Microsoft library and avast! certainly needs it (and the installer should put it there). I suggest to try to repair avast! from the Control Panel - Add/Remove programs applet; and if it won't work, download the installer from the web and use it to repair the installation.

I think Eddy’s suggestion is a good one… Did you try?
What version of avast are you using?

Similar threads:
http://forum.avast.com/index.php?board=2;action=display;threadid=1415
http://forum.avast.com/index.php?board=2;action=display;threadid=1415