press control alt delete and select task manager. under Applications select "new task" type in explorer and press enter - do you have your screen back?
If this doesn't work try:-
Unplugging any usb devices while computer is on with blank cursor. Then hard power off by pressing the off button and keeping it depressed until it switches off. Leave off for a minute or so. Leave usb cables unplugged, and power computer back on.
When you are trying F8 are you tapping it about 3 times a second as it is booting?