When you bought your computer you received a set of CDs containing the operating system (Windows 98, Windows XP, etc.) and possibly other software. You may not need them for a long time but when you do, you will need them right now! We suggest you buy a CD storage case, put all your CDs in it, and keep the case near your computer. In that storage case should be a list of any product registration numbers, user ids, passwords, and Internet access setup information from your ISP.
In the event your computer becomes a victim of lightning, fire, theft, etc. it may be covered by your homeowners insurance. Check with your insurance agent; if they have a copy of the invoice on file it may expedite your claim. Be sure to also give them the model number, serial number, date of purchase, and price for any hardware and software you purchase separately from the computer itself.