Purely guessing, rattling off a few things I'd look at if it happened to me, all in hopes it'll spark an idea or 3...

Vista being what it is, and trying to be practical, I'd first consider backing up the install drive to zip format files, plus export the 2 software sections of the registry, then restore an archived backup disc image -- assuming of course I had one from back when everything worked. Then with DVDA working, I'd do the updates etc. checking DVDA every so often as I brought the system back to current. If there isn't such a backup, and have to troubleshoot...

If it was working, then something changed -- the hard part is figuring out what. MS has updated Vista frequently, and one of those updates could have broken DVDA. Update history will show what's been installed, with a brief description. MS has also come out with several Vista patches & Hotfixes that aren't on Auto-Update -- you have to search either the knowledge base or scroll thru the list at the Downloads Center. Might look through updates, fixes, patches etc. to see if anything looks like it might have something to do with DVDA.

If you installed any other software, that might have caused the problem -- often you can rename a folder (only if nothing inside the folder is running) & that can work to confirm a conflict. Task Mgr is good for showing just what's running in Windows, and the free Autoruns from MS shows you exactly what's loading with Windows -- & gives the option to turn whatever on/off. Trying to start DVDA in Safe Mode should eliminate (or zero in on) some possible conflicts.

In extreme cases MS also offers quite a few, free tools from System Internals (a company that they absorbed) -- things like process and registry monitoring can show exactly what's happening as DVDA tries to start. The downside is that there's an enormous amount of data to wade thru, but it can and does work -- I've solved a few problems with software that wouldn't run in Vista that way.

While removing and re-installing software often helps, un-installation routines often leave trash behind, in both folders and registry keys. If there's a problem with registry entries, often the only way to really clean out keys/values associated with an app is to slog thru the registry using search. Tracking down folders can be a pain too in Vista since they've added virtualization -- something that used to be simple to find in Documents and Settings might be in any number of folders with Vista. When I've had problems where I needed to completely clean out other software (thankfully not DVDA) there was no shortcut to simply searching for stuff and deleting it.

One thing I'd suggest to bear in mind, and the reason I'd start off restoring a backup if it was available... Just installing something in Vista can have very different results depending on if the User Control BS is on or off -- it often determines where both registry values and user folders are stored. Software installation with the control off can cause some software (usually Vista compliant in my experience) to mess up the registry badly -- I've had reg entries that could not be accessed at all, including by the software that both wrote & needed to access them! And because the entries were there, they couldn't be re-written, or deleted [including with so-called super-admin privileges].

The only way I'd found to fix that sort of mess, and it has happened to me more than once, was to restore a backup... In fact a backup the registry (using erunt) & set a restore point before any installation now-days. MS has published another method that Thankfully I haven't had reason to try... so far...Because I have no idea how well it works (or doesn't) I'm putting it last instead of up top, 1st thing. If you set up a 2nd user with identical settings, supposedly the new user registry won't contain the same corruption or errors. Assuming it works you could then delete the original user account. If a registry problem isn't in the user areas (mine were not), I don't know if Vista in copying the registry over does some sort of error checking and repair or not.

Best Luck -- wish I could give a definite and quick answer.