Computer Boot Sequence

Question

We have an Evesham Computer and recently we have got a virus or bug of some sorts that is slowly taking over the computer. My nortons would not clear it and now ceases to work at all. So once again i chose to format my hard drive but this is not going to be as easy as it normally is, i have got my recovery cd ready and went in to my bios set up to change the boot sequence so the computer would boot from cd rather than the hard drive but nothing is highlighted like it usually is and so i cant change the boot sequence is there any other way round this.
Please Help