I found the culprit!

Turns out, it's all Avira's fault. I have been using it and PowerDVD together for quite some time, but it's the only thing on my system that can auto-update without my knowledge.

I reformatted again, and this time, the first thing I installed was PowerDVD. It launched, but would not play DVDs. I then went through a grueling process of trial and error, installing stuff, then testing PowerDVD, then rebooting, then testing PowerDVD, and so on. First came the LAN drivers. Then SP3. Then more Windows Updates. Then more Windows Updates. Then more Windows Updates. Then NVIDIA drivers. After that and a reboot, PowerDVD would play DVDs. After installing Avira, PowerDVD refused to launch. Disabling Avira Guard did not help. Killing the processes did not help. Disabling the startup items did not help. But as soon as I uninstalled Avira, PowerDVD worked again right away.

That's what I thought from the start, conflict with another program, glad you finally found the culprit, an update of your anti-virus software, all you need to do now is find another anti-virus program.

Robert Intel i7 930, 16GB ram, Radeon HD 5770 1Gb,Ver. 14.12 Win7 64 bit
Intel i7 7700 HQ, 16 GB ram Nvidia GTX 1050Ti 4GB dual drives 1 TB SSD + 1 TB HDD Win 10

PDtoots

In case anyone else has this problem and is reading this (it shows up on the first page of a Google search for "powerdvd wont start" now), I've been notified of a fix by a member of Avira's support forums.

Go to Configure Antivir > tick "Expert Mode" > General > Security > Uncheck "Protect processes from unwanted termination" and "Protect files and registry entries from manipulation" > Apply > OK. Reboot and then check if your PowerDVD 7 works now.

In my case, I only had to uncheck "Protect files and registry entries" and reboot. But I do wonder just what PowerDVD is doing that could be interpreted as attacking Avira's files and registry settings.

I have Avast and have been going crazy with this problem for days. FINALLY we have identified the culprit...anti-virus SW. In my case the problem did not appear until last week so I suspect that what ikansplewurdz saw in Avira made its way into an Avast update in late January.
As with his solution, I was unable to simply disable the Avast to get things to work. We were unable to come up with a combo of excludes so we uninstalled and disconnected from the Internet to protect the computer until Avast addresses this issue.
My deep thanks to all who have worked on this solution.....

We may have found the file that "violates" the AV programs "C:\Program Files\CyberLink\PowerDVD8\VideoFilter\cldorz.dll";
"Trojan horse PSW.Ldpinch.ADYW";
"Moved to Virus Vault"

Avast fixed things for this file but there is still the same problem, resubmitted a request....

solucion para avast :
1º escudo de comportamiento
2º opciones avanzadas
3º desactivar casilla "monitorizar el sistema en busca de modificaciones no autorizadas"