Photoshop Questions

Hi, I'm still new to Photoshop, and I'm experiencing a little inconvenience here. I would like to know why can't I use some of the filters; actually most? And How do I undo more than one time then redo it back? BTW I am using CS3 Extended. I hope whoever knows this can help me. Thanks.

History: to undo more than one time you can use Ctrl + Alt + Z or go to Window -> History. this will open a a section in the sidebar where it shows the recorded history of your actions which you can click on to revert back to that time.

As for filters, it varies depending on the situation. It could be because your layer is not selected on a rasterised layer or because it is a locked layer. A printscreen would help.

Okay, thanks Safan.