My Office Button shows Recent Documents for a short time then the documents go away, especially when I exit Word – gone.
In Word 2007, I clicked:
Office Button > Word Options> Advanced> Display: Show this number of Recent Documents: 10
Did the same thing in Excel 2007. This does not work.
— Submitted by Jim Lennon from St Paul, MN, USA
You used the correct method for changing the number of Recent Documents displayed in the list. (The default is 17, but you can change it to any number from 0 to 50.)
In Office 2007, the Recent Documents list is written to the Windows Registry. So I suspected that something outside of Office 2007 was causing the problem. Because I never ran into this situation before, I went hunting for an answer.
What I found out is that your anti-virus software can cause your Recent Document list to disappear if it is set to clean the list at regular intervals. The fix is as easy as disabling the setting in your software. 
So check your anti-virus, registry cleaner, or privacy program settings…one of those is most likely the culprit.
1. OFFICE 2007 (Word, Excel, PowerPoint) Losing Recent Documents List. Discussion Groups Home. Microsoft Office Online. [Online] Microsoft, June 18, 2009. [Cited: November 10, 2009.] http://www.microsoft.com/office/community/en-us/default.mspx