If you reformat the hard drive ( I assume it is the 'C'' drive you are talking about) you will erase everything including the OS. You will then have to reinstall the operating system on the clean drive. all data will be lost.
since you are using Win XP why not try to use the System Restore feature available through the run, then msconfig command. IF that doesn't work you can try to repair the OS from the XP disk by selecting repair when prompted. If the drive is really screwed up you may want to risk a reinstall over the current operating system which may preserve you data files.
If you have an other configuration --multiple HDDs or partitions, etc. please let us know.
Guys please,help I know this is an obvious question for the PC person but I don't know how to do it, and I have untill morning to fix this PC. I have to reformat this hard drive and erase everything to get back my Regedit files (the command prompt won't work because of messed up files) that have gone bad and won't take commands. So hopefully if I reformat then that will help my CD/DVD rom drives to read disks (they spin but won't read).I've gone back and forth with dell customer service and they won't help me unless I pay $100 for some help that I could get off forums. I REALLY need this help, please.