Hi Jackhitz,
I assume you mean you want the operating system and application software on the C drive and all your data ( documents, pictures etc. ) on the B drive ( setting the B drive to the active one will probably make your pc un-bootable ). Do the following ( I'm writing this from memory working on an XP machine so it may not be exactly what you see but I'm sure you'll get the general idea ):
open windows explorer and click on Documents under Libraries
right click on My Documents and click on Properties
click the Location tab, change the location to the B drive and click on OK/Apply
click OK/Apply/Yes to This folder doesn't exist, would you like to create it ?
click OK/Apply/Yes to Would you like to move the contents to the new...