I use opcom and it has sorted out alarm problems on Astras - I got mine from a bloke on vvoc, it's a bit more expensive but you get first class support if there's any problems. His website is http://opcom.weebly.com/ - the problem most people have is downloading the software and installing it. Some of the ebay versions don't work. On this site you download the winrar file, then the opcom file, then the trick is to manually set up your device manager to get the USB connection.
(When I say manually - I mean you need to point the installation wizard to the opcom folder, don't let it try to sort it out itself.) However, he usually will do the installation for you remotely if you run into trouble and can't get it to work. So it's worth paying a wee bit more to know you aren't going to be stuck with a none working product.