Edit with Notepad or any other ASCII text editor the file "C:\Users\username\AppData\Roaming\Quicken\config\QUSER.INI" or "C:\Users\username\AppData\Roaming\Intuit\Quicken\config\QUSER.INI" for older Quicken versions. Replace "username" in the path with your Windows account name.
In this file you will find lines looking similar to these:
[RecentFiles]1=C:\Users\username\Documents\Quicken\test2=C:\Users\username\Documents\Quicken\UKR3=C:\Users\username\Documents\Quicken\UKR2005-20064=C:\Users\username\Documents\Quicken\Ukr2004