How can I undo an order? I've payed and dowloaded "Limewire 4.16.6" but it won't install on my Mac OS X 10.3.9. Can I get the money back?

No. LW doesn't offer refunds. Lime Wire » Refund Policy

Try going to your Pro d/load page and get one of the older versions. There should be a link to "Older Versions" on that page.

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If it ain't broke, fix it till it is!

Best Thing is: never to buy anything by means of the Internet! It's a mousetrap

Yes, just blame the Internet for your lack of research before buying Pro. Like e.g. maybe trying to see if the free version would install on your comp.

And BTW Limewire should be possible to install on whatever computer given that the computer had proper software installed beforehand and was configured correctly.

Don't get me wrong here. I too believe that Limewire isn't a very good program, but by that I mean the program itself, not not being able to install it.

I'm not getting any help here; just accusations (sorry my English)

Actually you started with accusations when you said anything bought of the Internet is bad. A statement like that deserves a counter statement, which is exactly what happened.

Millions of people have no problems buying programs and other things of the net, so don't blame the Internet for your lack of research.