So, this isn't a problem of not being able to open a book after downloading the acsm link. I can't even get that far!
When I click a download button which normally leads to an acsm download, the browser (have tried FF, IE and Chrome) tries to open the link in a new tab. After a few seconds it fails to find the 'website'. If I right-click and choose 'Save Link As', the browser attempts a download of the URLlink.acsm but the download fails because it can't find the source file.
I'm having this problem with new books and ones which I've previously purchased and downloaded with no trouble. I've tried disabling the pop-up blocker, the firewall, the anti-virus software and everything else I could think of.