Addressing the reconfiguring only, that comes from running multiple versions of the software on the same computer. See the registry hack at Toolbars and Word 2007/2010 - dual version registry hack.

On the spacing of the .doc file that works in Word 2003, take a look (in Word 2003) at the document template dialog under the Tools > Templates and Add-Ins. Is the box to update styles from the template checked? If so, that may be the source of your problem; uncheck it and then try in Word 2007. What happens when I send my document to someone else? Will Word mess up my formatting?

Note, though, that even if it is the same printer, different operating systems have different drivers and can produced different results.