Windows disc shipped with vostro 3750 windows 7 pro

"Anyone know if the disc shipped with this lap top has both 32&64 bit os on it..?"

ONLY the version you ordered the system with.  Retail media includes both; OEM media includes only one.

"I ask as I plan on upgrading to windows 8.1 and would like to use the w7pro on my older computer that's running xp..  but it needs to be 32 bit.. and I have yet to find out if the disc is both formats,  I upgrade the xp one to w7 and skip windows 8 altogether if I could find w7 to buy..  but all the stores and even Microsoft doesn't set it anymore.."

You can't upgrade an XP machine to Windows 7 using Dell media.  The Dell media should only be used on machines that have an accompanying license (or qualify for DOWNGRADE rights).  Windows 7 can still be purchased, just not from Microsoft:

http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&Description=windows+7+32&N=-1&isNodeId=1

Why if you are moving up to w8.1 couldn't you use the lic w7 on an xp machine,  both that I bought and paid for from dell?  other than the laptop being 32 bit.. (so I've been told) inferon  1705 .

why would I need to buy another lic copy.. as it not be used on more than one machine, just not the original one

Software installed on a machine from a PC manufacturer/builder is called OEM software.  You obtain that software for a fraction of the price of buying it for full retail price.  One limitation of OEM software, that comes from the lower price you paid, is that it is tied exclusively to and lives AND dies on the machine with which it shipped and was licensed and cannot be transferred to another machine.  Sell, donate, or throw away your computer, the license goes with it.  "Retail" software CAN be transferred, OEM cannot.

Microsoft have just removed download links for Windows 7 and Windows Vista from Digital River moreover their badly thought out Windows 8.0/8.1 downloader does not give the option to select 32 Bit or 64 Bit and in the case of your system will install the 32 Bit only.

The only way to switch from 32 Bit to 64 Bit via downloadable solution is to use the Enterprize trial evaluation:

http://technet.microsoft.com/en-gb/evalcenter/hh699156.aspx

You may use this to clean install Windows 8.1 64 Bit and then purchase Windows 8.1. The downloader will then download 64 Bit installation files which you can use to make a .iso for backup purposes and a bootable USB.