Reformatting your computer will wipe it clean of everything on it, even Windows. Then you will have to have Windows or some other Operating System (OS) to put on it or the computer will do nothing. From what you are explaining you are going to have to either find one of your previous Windows CDs that you had or buy a new one.
To reformat your computer you will need a way to start your computer using a different OS than Windows. If you have a floppy diskette drive on the computer, you can make a boot diskette that has DOS on it. Then you can start your computer in DOS and format the drives.
If you only have a CD drive, you will need to get a boot CD that has an OS on it. PEBuilder (freeware) can make a boot CD with the BartPE OS on it. It has the files on it to format the drives. But for BartPE to work, you must have the I386 folder in the Windows folder and the files that come with it.