to "start fresh" you will need to reformat your hard drive. the only risk associated with reformatting is that you lose any and all data on the drive. but if this is something that you are ok with, its not a terribly difficult process. first you will need an installation disc of whatever operating system you are looking to run (assuming you want to stay with xp then you will need a windows xp installation disc). if you do not have one of these discs, you can either try to buy one or borrow one from someone, as long as you make sure to get the product key that goes with it. if you cant get a hold of an installation disc, you won't be able to reformat your hard drive and reinstall the operating system.
if you do have the disc, first make sure you have any data that you want off your hard drive backed up onto a flash disc or external hard drive or something. next you will need to put the disc in your disc drive while the computer is on. close any auto-run windows that pop up, and then restart your computer. when the computer is first starting up (before it reaches the windows xp loading screen), you will need to press the appropriate key to bring up the boot menu. this is usually the escape key, or one of the function keys (usually f2 or f10 or f12), it happens quickly, but the correct key is shown on screen, so just look for that as the computer starts up. it is usually displayed in one of the corners. once you bring up the boot menu, you will need to use the arrow keys to select the choice "boot from cd" and press enter. it will prompt you to "press any key to boot from cd...", press any key. next it will load the installation software and then bring you to a windows xp installation screen. you will need to follow the stops on screen to install a fresh version of windows. when prompted to choose an installation directory, be sure to follow the instructions on deleting partitions, as you will want any existing partitions deleted so to provide the maximum amount of hard drive space. once you have deleted all the partitions, you will need to format the drive, it will ask you how you want to format the drive, i would recommend that you format the drive with the NTFS file system, but don't do the "quick" option. then, from there out, the installation takes over, and you will be walked through all the steps needed to install windows.
the only hassle you might have will be finding drivers for some things like your network interface controller and monitor resolution, but those can be found online or on discs that came with the computer.
hope this helps, feel free to contact me with any questions.