Laptops are usually built as an enclosed system, difficult to even open the case to even begin looking for the CPU. You get a CPU choice when you customize a laptop so I assume you can upgrade with some difficulty, but I don't know if you'd have any success running Windows 2006 (does that even exist?? 2003 is barely even out yet?). Especially since you can't upgrade the processor beyond the line the laptop was made for, and it's already pre-owned so it's at least lat year's processor lines.
i.e. you might be able to replace a low-end Duron with the top of the Duron line, if the FSB is supported, but you can't replace a Duron processor with a P4, Athlon 64, or other next-gen processor.
I still haven't located the tool needed to open my sister's old laptop for servicing.