Is your website ready for Internet Explorer 7.0 ( IE7 )? If you aren't ready or your not sure, then please checkout the Checklist created by Microsoft to see if your website is compatible.
If your site isn't ready, then you should consider updating it soon because Microsoft is going to push out IE7 as a High Priority Automatic Update soon. This is rumored to happen sometime in November ( which is very close ).
You can install IE7 from Microsoft's main Internet Explorer Home. You might consider installing IE7 on a machine other than your development machine to test with first. Other than using virtual machines, I don't know of a good way to have both IE6 and IE7 side by side on your machine.
For those of you who like shortcuts, here is a Quick Reference Sheet for IE7.
Note: If you do install IE7 as your primary browser and you have difficulty with other websites, that haven't been as meticulous as you to make your site compatible, then you can install the User Agent String Utility version 2 developed by Microsoft.
"This utility changes certain registry keys on an IE 7.0 installation to allow a user to emulate an IE 6.0 installation as a simple workaround until problematic sites update their code. It also offers users the option to report these problematic sites to Microsoft."