I use acer aspire 5100 but my orbicam is not working

Try reinstalling the Acer Orbicam drivers from http://support.acer-euro.com/drivers/notebook/as_5100.html .
As you don't say what operating system (XP or Vista) you are using I can't give you a more precise link.
If you use XP, try the "Bison" driver.
If you use Vista, there are three drivers (Suyin, Logitech and Bison). One of them is the right one, I don't know which one, as it depends on what exact camera is in your computer. But simply try installing one at a time, until you hit the right one.