How do I get into a locked desktop to change a theme image in winxp?

My desktop somehow became locked and instead of the theme I had,it now has a plain blue screen.Ithen tried to change this to another theme by adding an internet picture as a backdrop image,and still the same blue screen.When I tried to change it thru the display in tools,I found I was locked out,and I don't know exactly how this came about!How do I get into a locked desktop to change a theme image in winxp?
Brother, you just COMPLETELY stumped me, umm the only thing I can think of is you are on a limited account and an administrator denied you access to change the theme. However, no desktop makes your computer go much faster and I personaly don't see the point of it.

arledgetd (is stumpted)

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