You can reuse your Windows 7 license key when installing Windows 7 from the same disk as you used for your Dell if (a) it was a retail version, and (b) you can convince Microsoft that the machine it was installed on is longer in use. So that's free.

Same for MS Office. Belarc Advisor would have been able to tell you the key, if you had run it. You can try to boot from the old disk in Safe Mode in some other PC. That might work enough for you to run Belarc. It's a small chance, but worth a try.
It makes some sense to save product keys you paid for. But that's too late now.