Help! What might happen if Excel spends a lot of time pasting?

Hi all,

I just don't understand this.

I don't know ever since when, my Excel is so slow, esp. when I copy
and paste.

When I copy from one book, and paste to another book,

the very instant when I enter "ctrl+v" to paste...

the whole Excel freezes and my CPU usage went to 100%...

The whole program is not responding any more.

Can anybody help?

Thanks!