64-bit versions of Windows do not include the required database drivers to run Portfolio Manager in 64bit mode. From v1.8 of Portfolio Manager the database format has changed to solve this issue. When first loading your portfolio in v1.8, your database will be automatically upgraded to work on all versions of windows. Unfortunately this upgrade will not work on 64bit windows as it cannot load the previous database.
If your currently running an older version of Portfolio Manager and wish to run it on a new PC with 64bit Windows then you will need to upgrade Portfolio Manager to v1.8 on your old PC and load your portfolio before transferring it to your new computer. This only has to be done once. If you do not have the old PC please contact support to get your portfolio upgraded.