honey and sting

Rundll32.exe is a normal part of Windows. Most of Windows (and most of all Windows applications) live in "Dynamically Linked Libraries", known as DLLs. If Microsoft's engineers ever needs to fix bugs in Windows, they can do so by shipping one or more new DLLs. Run-DLL is the tool that Windows uses to execute these.

So, what's happening is your antivirus program is using one or more DLLs, probably one of the Microsoft libraries, and then doing a lot of processing on its own. So Rundll32.exe comes up looking like a CPU hog (as the DLL is doing a lot of work reading files), and then yourt antivirus looks like another one as it analyzes the data coming back through the DLL.