If you buy from B&N then your best bet is to get an ePub, because its DRM is easier to remove than from eReader. Currently, it seems you need to download the B&N ebook from within Windows B&N Reader to get the ePub version (you might still get the eReader version, but it is worth trying). The ebook will be in My Documents\My BN eBooks\EMAIL. Then see
Barnes & Noble DRM for EPUB circumvented.
Note that B&N is the only ebook store currently using "password" DRM for ePubs. If you buy ePubs from any other store they should work "as is" on your PRS-300.