Use SmartSum as a replacement for the woeful Windows calculator. The program has a mile of features; the most useful is that you can set the program to calculate GST automatically. Other features include: optional vocalisation (which prevents errors); tally roll tape so you can see your calculations as you make them (you can also save and print calculations, tick them off, and add notes to them); three user-defined tax rates (set one of them to the GST); 10 user-defined macros; conversion macros; correction and automatic recalculation of figures; 10 memories; the ability to title, save and reload calculations; interfaces with most popular spreadsheets; and much more. The macros feature goes beyond mere recording of keystroke macros, as you can write your own macros and distribute them to others. The program is advertising-supported and free. A complete Help file is included.