AT & T DSL crashed my work computer - .exe nonfunctional

:boxing A T & T recently upgraded their DSL service and the upgrade conflicted with ZoneAlarm firewall and virus protection. While on the phone with A T & T , I followed their instructions to make the two systems compatible. In the process, all of my programs quit working. None of my programs are functional including the control panel.

The only thing my computer will do is connect to the internet. I want to format my C drive and reinstall Windows XP. I've tried to format with windows format command. That doesn't work. The only way I can format is to be able to get to the C: prompt and enter the "format C: /s" command.

However, even when rebooting, the only option I have is to press "Del to enter setup" which takes me to a CMOS utility program. I am unfamiliar with that program and do not know the options to pick to get me to the C: prompt.

When my computer boots, the option to press any of the "f" keys is not available. Can you either tell me what to do in the CMOS utility program to be able to get to the C: prompt or tell me another way to get there. Once I get to the C: prompt, I know how to format the drive and reinstall Windows XP.

Thanks for any help.