kruser,
Adding to the previous comments:
1. You are correct, you have to close all windows and documents that refer to the table you're trying to restruture. I know it seems counter-intuitive, however, Paradox detects indexes when you open tables. If you restructure the table, the indexes may change and the view that's currently open may no longer be appropriate. (For example, you might delete the active index). Thus, to ensure that your data is always viewed properly, Paradox requires exclusive access to the tables being restructured.
Now, having said that, there is one condition where you can restucture an open table. If the table is only open in a single Table window, you can choose View | Restructure (I believe) to restructure. However, that's not terribly useful for you really shouldn't be using Table windows to work directly with your data except for purely (and brief) administrative tasks.
2) Just to clarify one minor point that Mike made: Paradox 5.0 was written for Windows 3.1, not Windows 3.0. It's a subtle (and, at this point, moot) distinction, but I do not believe the program will actually work under Windows 3.0.
He is correct in that you really should upgrade to a more current version. Version 11 is now available through WPO 2003 Professional. While it hasn't really changed since version 10 (save for bug fixes), it has changed rather dramatically since Paradox 5.0. There are numerous bug fixes, new features, and other stuff that will make your Paradox work much easier.
3) In addition to the web site Mike referred to, you may also find these helpful as well:
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4) Paradox can be rather confusing to get the hang of, however, if you stick with it, it will make sense. While there are certain decision decisions that we could argue about, nearly all design decisions make sense when fully understood. I remember my learning curve and it didn't make sense for a very time. However, once I got the hang of it, it really made sense. Your mileage may vary, of course, but that's why you have such helpful folks slouching about areas like this and creating websites like the above.
Hope this helps...
-- Lance