When you open an older Quicken data file in a newer version of Quicken, because of the difference in the newer program's architecture, your data file must be updated, or converted, in order to be read correctly in the newer version of Quicken.
If you are converting from a much older version of Quicken, it may be necessary to use an intermediate version in order to safely convert your data file. (more information).
When you first start Quicken for Windows, it looks for the last Quicken data file you used and walks you through the conversion process. If you have additional files on this computer to update, simply attempt to open the data file in the new version of Quicken and it will ask you if you would like to convert the file.
When you update a file, you will no longer be able to use it in the previous version of Quicken. However, Quicken backs up your original data files in the same folder as the converted files in case you need to open the file in the older version. If necessary, you can find them in the Quicken\QXXFILES folder, where XX equals the last two digits of the number of the Quicken version you used before conversion. For example, if you are converting from Quicken 2004, the subfolder is named \Q04FILES. If you are converting from a version of Quicken that is older that Quicken 98, look in the \QW6FILES folder for Quicken 6 files, in the \QW5FILES folder for Quicken 5 files, or in the \OLDFILES folder for files from all other versions of Quicken.
Instructions for importing Quicken for Macintosh data into this version of Quicken for Windows can be found here.
Instructions for exporting Quicken data for Windows to Quicken data for the Mac can be found here.