In what situation do anti-virus programs intertwined into a loop?

When you simentanuously scan your computer (ex. McAfee scanning for viruses and spyware and Spybot Search %26 Destroy scanning for spyware) , if both software encounter the virus/spyware, they try to delete it. But both software request access to that sector of the hard drive but can't access it because the memory can't be accessed. They try and try until the software crashes. This will be seen as a "Not Responding" in your Windows Task Manager.

In what situation do anti-virus programs intertwined into a loop?adware remover

Not sure what you mean by "intertwined".

As for "loop", I will presume that you are referring to the virus-infected file's re-appearance after you have deleted it.

Some virus programs conceal themselves on your hard drive and in memory. If a running copy detects that its file has been deleted, it can create another file of itself from its own image in memory.