I've spent the last few hours working on how to get the Sonic DLA driver uninstalled and, finally, it's gone ! I've tried all sorts of things like registry edits, deleting directories, deleting dll's, using the Vista DVD to restore once I got a Blue Screen but nothing worked. The final solution turned out to be relatively simple and didn't require any registry hacking at all. It went like this ...
1) If there's been any manual hacking of files and/or registry, use System Restore to get things back to how they were before it started.
2) Download safemsi.exe from here. The utility lets Windows Installer be invoked in Safe Mode.
3) Boot into Safe Mode by pressing F8 before the Vista boot screen starts.
4) Run safemsi.exe.
5) Remove the offending application as normal using Control Panel.