The cheapest and easiest way to have multiple users access

On the same machine? Different machine? Simultaneously?

Do what?

Either would be a downgrade and cannot be done.

No.

You want both users to be able to use QuickBooks simultaneously, accessing the same file? You would need a 2nd QuickBooks license. Check the QuickBooks help file under "multi user".

Both users need to use the same version of QuickBooks, in order to access the file. You may be able to purchase a 2nd license directly. I don't know whether it would be cheaper to do that or to purchase a second copy of QB Premier 2005 from a retailer.

???? Windows XP *IS* the upgrade. Windows 2000 and Windows 2003 are older versions of the operating system.

Pro? I thought you said you were running Premier?

If both computers are running the same operating system, and if the same version of QuickBooks is installed on each computer, and if the computers are networked, then yes, you should be able to have two users access the file, provided you have set up the program to be multi user, and you have entered two licenses.

The help file within QuickBooks has all the information.

Depending on the cost of the new license (or 2nd copy of the program), you may find it more cost effective to buy two new copies of the 2007 version. The Intuit "sunset" policy means that the 2005 edition will be unsupported after spring, 2008. Not supported in Intuit parlance means certain vital portions of the program will stop working.

It is up to you to determine whether the cost of adding a license to the old