anInvoicer is free for use as an Inventory system. It is not timely limited, it will never expire. Only difference from the registered version is that Invoices whose total amount exceeds $100 can not be printed.
anInvoicer require Windows 2000 or Windows XP. Other than that, there are no special requirements to run anInvoicer.
You can download anInvoicer here (3.5 MB).