You have two simple choices to format your HD. The first is by, as you say, going into DOS mode. To do this when you first switch your computer on keep pressing the F8 key until you get a small menu which asks you how you want to enter your computer? Look for the sentence with command prompt and select that. The computer will then do it's thing and end up settling at the MSDOS prompt. You can then do a format using the XP cdrom. The only thing is that I think from memory the command is not format but oformat. So you would type in at the c: prompt oformat c: the computer will ask you yes or no etc and once you decide yes it will format your hard drive. The other way is by going into your bios and setting the boot up sequence to start with the cdrom. To do this hit F1 or del key when computer asks you to and that will take you into your bios. Once you have saved and exited, make sure you have the XP disc in the drive and the computer will automatically boot into the xp install menue and all you have to do is tell it what you want? Tell it you want a fresh install and follow the prompts. You will eventually end up on the screen that asks you to format your hard drive etc. All you have to do is tell it what type of format you want ntfs or fat etc
Don't want to tell you to suck eggs but I hope that helps?