Here I go with the metaphors, fryke!
If you lost the keys to your car, I'd be willing to bet you'd be a bit more frantic in finding them... it's the same issue with your computer. By not keeping the original install/restore CD/DVD around, you've effectively "locked" yourself out of doing certain things (and in certain circumstances, you can completely "lock" yourself out of your computer unless you have the install/restore CD/DVD).
Always keep a set of installation CDs/DVDs around. Treat them with the same oversight and carefulness that you treat your car keys, because they are, in essence, the "keys" to your computer, and you will encounter a situation where you cannot do something without them (like this very situation).