A good strategy would be to make use of the new system just as you bought it.
Then you can plop the old drive into the new system as an extra, non-primary, drive (for example; instead of the "C" drive the old one would become "D") , and all of your old data files and documents will be available to the new machine.
The manuals for your new system will be able to give you good instructions on how to mount an additional hard drive inside it, and (if it is a good manual) what you have to fiddle with in Windows to make it work just the way you want it to.