What was the first MacBook that people were able to install Windows on?
Apple introduced Bootcamp in 2006 as a beta for OSX Tiger. (source)

So it's since 2006 that the Macbooks are able to install Windows. However a support note specify that the oldest Macbooks aren't compatible with Windows 7. 

NB: Notice that laptops powered by PPC processors were called iBooks &amp; PowerBooks. Macbooks (Pro) are only powered by Intel processors.