Did you use a Dell CD?
Do you have a corp license? They're different.
Was it XP Pro? Also often different.
I can tell you for certain that number will work fine with a non-Dell XP Home CD for the install but you won't be able to authenticate. If you install from a Dell CD it doesn't even ask for the license number. Take a Dell with XP Home and run a program that recoveres the license number. It won't match the one on the case - but that number won't work either with a non-Dell disk.
Since my clients never have the disk I tell them, here it is, it runs, you have 30 days go get a CD from Dell or is stops working.