I'm not sure what "rotating a circle" means exactly, but
here's an article on using threads in applets. There are also a number of example applets on
java.sun.com, one of which -Animator- specifically deals with threaded applets.
Regarding signed applets, you'll find information and further links in the Applet FAQ, which is linked in my signature.
[ October 23, 2005: Message edited by: Ulf Dittmer ]