Advanced Accounting 5.1 runs under one of two "modes" - DOS or Windows. This has NOTHING to do with whether your PC is running under Windows or not.
When Advanced Accounting 5.1 was originally released in late 1996, the bulk of the user base was still running DOS or Windows 3.1. So as to not require users to update their hardware and keep the new system as compatible as possible with the prior 5.0 version, the Windows mode was made to be cross compatible at the program level and runnable under Windows 3.1. For most existing users, this was a large benefit.
However, this means that the Windows mode of Advanced Accounting has the same limitations as Windows 3.1 and in general has a lower resource threshold than does the DOS mode.
We have done everything possible to ensure that every program option/feature is available and works the same in both modes even though this adds considerable additional development time.
However, the Windows mode in the current version is best suited for special uses such as printing to a printer that requires a Windows printer driver and can be printed to from a "DOS" application. For commonly/heavily used data entry programs (for example: sales order entry) we do not recommend use of the Windows mode as it is less forgiving of user error, conflicts with other software, and resource consumption. In fact the DOS mode is the robust mode of choice for Advanced Accounting 5.1. The DOS mode in fact has significantly more resources available to it than does the Windows mode and runs under all operating systems released by Microsoft to date (including XP). In fact the DOS mode of Advanced Accounting 5.1 and Windows 2000 is the best possible combination, all other things being equal.