Suggestion:
Start with a new, fresh, clean bootdisk. DO NOT create the boot disk from the machine that is giving trouble. If necessary, go to your good machine and download a boot disk from bootdisk.com. Make sure the boot disk is write protected BEFORE you attempt to use it. Boot up the problem machine and fdisk and format from the new boot disk. Reinstall Windows from an original Windows CD (no copied CD). Scan for viruses on any and all disks you are using to install or restore progams from.