Exposing functions to Windows and other programs

Hi All

Is there a way to setup a program so that another program can tell it what to do?

We have two programs, one that manages leases and one that manages accounting. Items in the accounting program correspond to items in leasing program, ie a payment for a leased item in the accounting program will have a matching item in the leasing program.

In our perfect world we would like to be able to click on a payment in the accounting program and have Windows switch focus to the leasing program, and have the leasing program open up to the corresponding leased item.

So does anyone know we can make a program expose functions to Windows that another program can call?

Thanks