<< but what about software? >>
Just like with hardware it varies from product to product. You often don't get a manual, you sometimes have no right to support ("contact the hardware/system vendor for support of bundled products"). You often don't qualify for version upgrade rebates and sometimes don't qualify for upgrades at all.
When buying an OEM OS the main thing to check is that it inlcudes "media" (the CD) as well as the certificate of authenticity (COA) license. Some vendors sell just the COA to people like small businesses that need extra licenses but not more copies of the CD.