an OS is an operational system... windows 98, windows 2000, windows xp........ ok this is what i did...... if you have a startup disk (floppy), put it in your computer and restart, when u restart, it should recognize the disk. then it will take u into DOS..... then once u are in DOS, insert a OS CD if you have one. once it is in, type the name of your CD-Rom drive. its usually E:/ or D:/ . once you typed that, press enter. then type setup.exe and then follow the step-by-step instructions.