How to find keyword search engine results from previous weeks?

[CreateForLess]

Does SEOMoz archive that information somewhere?

[SEODinosaur]

Yes. You must create a campaign with your keywords to be recorded and after every week if you scroll over the keyword, two small images will pop up the first is for keyword tracking and the second is for top SERP sites for that keyword.

Hope this helps.

[NakulGoyal]

Do you have weekly/monthly reports automatically generated ? If yes, you could download them from your email if you still have them. If not, you could also click on any particular keyword in your campaign and see historical rankings data for that keyword. I hope that helps.