A number of reputable online retailers such as Amazon.com still have
full (not upgrade) copies of Office 2003 available if that's what you
want.
If you install both versions:
You can only have one version of Outlook at a time in a Windows
partition. Word may spend a few seconds to a few minutes reconfiguring
itself each time you switch between 2003 and 2007 or vice versa. You
can't use Word 2003 as the email editor in Outlook 2007, and you can't
use Word 2007 as the email editor in Outlook 2003. The "send to mail
recipient" option probably won't work in 2003 programs if you install
Outlook 2007. Check whether your computer meets the minimum system
requirements for Office 2007. I think these are the most common issues
with having 2003 and 2007 on the same computer, but I'm sure there are
others.