well, you can do a couple of things
1. go into your BIOS and get your Service Tag # and contact Dell (you must be the registered owner)...they should be able to give you the key for your system...there might be a charge though
2. you can wait until Windows 8 comes out in a couple of months and purchase that, Windows 8 is better/faster and I think smaller than XP anyway
BTW: what I did when I got my system was to cover the sticker with a piece of clear tape to prevent it from wearing away, But it doesnt really matter now anyway because I am dual booting Windows 7 Ultimate and Windows 8, but if I ever do want to sell my system I can use my restore CD and my original XP key is still on the bottom of the system.