How to change boot option

Hi, I am using Dell Inspiron 1545.   It has Vista Home Premium.

I have just enrolled into a course on Linux.  I dont want to have any dual-boot on my laptop.

I have a Fedora-5 bootable disk, frm wch I want to boot and practise.

But I am not being able to get into the BIOS to change the setting as ' Boot from CD'.

At the time of booting, the screen is showing ' Press F12 for Boot options'

I am pressing Fn +F12, but I am not being able to get into the boot options. Evry time Microsoft Corporation is coming....

pls suggest me on this issue.