I am running Vista Home Premium.
RegVac was running fine up until this past Saturday.
When I attempted to run it Saturday night, before shutting down, it finished the classes cleaner when a Windows popup said RegVac had shutdown and Windows was searching for a solution to the problem.
When I hit the cancel button, it closed the popup, but it also shutdown RegVac.
I looked for an update and found RegVac, version 5. I installed it and the same thing happened again.  
So I uninstalled RegVac completely, deleted the program folder and used regedit to remove the one remaining registry entry.
Then I rebooted and installed version 5 again.
The same problem came up again.  
I thought maybe the delay between cleaners was causing the problem, so I changed to expert mode and ran the classes cleaner.
Everything was fine, until I clicked the delete checked entries button.
At that point, the popup showed up again and shut everything down.
Looks like a problem with user account control, but it doesn't ask if you want to shut down the program, it just does it.

   Other people have reported that too.  I need to figure out why that is happening.  Did that happen in versions before version 5?  And has it always happened in 5?

This was a sudden change. As stated in the first post, I was running version 4 with no problems at all until this past Saturday, then this problem suddenly showed up.
I will try running the program with user account control turned off and see what happens. I am fairly sure that is what is causing the problem.

Previously, I had a problem where Internet Explorer 7 would not download anything from the Internet. I contacted Microsoft support asking how to do a reinstall of IE 7.
I finally had to do an overinstall of Vista to correct the problem, but not before having to uninstall Norton 360 at Microsoft's insistance. They were convinced that the Norton program was causing the problem even though I told them that all I had to do to download something was turn off Microsoft's UAC.
After I uninstalled Norton 360 and the problem remained, they admitted it was caused by Vista and had me reinstall it to solve the problem.

I am surprised. I turned off user account control and the popup window still came up and killed the program.
Popup window insists that some problem has caused RegVac to stop working and it is looking for a solution. I let it [popup] run all the way through this time and it stated no solution found and then terminated the program.
RegVac did get a few steps further with UAC turned off, but still less than half way.

OK, I will be out-of-town today but I will try to get to this Saturday.  Hopefully, I will figure something out.  Thanks for your help.

More information. I tried turning off Vista's data execution prevention for the RegVac program. This had no effect on the problem either.

I am not able to reproduce that.  I also suspect the UAC settings.  I tried changing the ones that I thought would affect that and I could not get RegVac to mess up.

There are no settings changes available in Vista Home Premium for UAC. So I assume Microsoft has set defaults for these.
Researching this, I discovered you need secpol.msc snapin for administrative tools to enable setting of local security policy. This doesn't exist in system 32 in the Windows folder for my current install.

   I tried RegVac in Vista Home and could not get it to mess up like that.  Someone else is reporting the same problem in Vista Ultimate.
   My guess is that it has to do with an entry that RegVac is trying to remove.  You could go to the Automation section and run individual parts of the Classes Cleaner and hopefully narrow it down a little more.  Let me know what you find.  Thanks for your help.

Your assumption would appear to be correct.
I was able to run everything in the classes cleaner except associations.
On that scan it stopped before completing and the pop up box came up.
To be sure there was nothing else, I also ran and was able to complete all the other cleaners.

RegVac was stopped after finding 15 entries in associations.
The last entry found was
HKey_classes_root\SAP1.SpPhrasBuilder\CurVer

I am using Windows Vista Ultimate and having the same problem with the Classes Cleaner.

I found that unchecking  
HKEY_CLASSES_ROOT\Local Setting\Software\Microsoft\Windows\Shell

allowed the program to complete it given task even in novice mode (you have to move fast to uncheck it as it goes by)

Try to copy or type the HKEY string into a redo.reg file in your reg vac directory - Go to Help and see Keep Bad Keys for detailed instructions.  

I hope this works for you

PS I tried again this morning and after 2 successful operations on previous days  the redo.reg did not pick up the Key and I had to manually uncheck the key. Once unchecked everything worked

Is their another way to keep the key from being detected as bad?

I'll give it a try. Don't remember if that was one of the 15 items or not.

   I guess another way of doing the above recommendation (about using the Automation section) would be to go to the Class Cleaner section, click on Find Bad Entries, and then uncheck the items you don't want to remove.

Unchecking that key[HKEY_CLASSES_ROOT\LocalSetting\Software\Microsoft\Windows\Shell]
did not work for me in Home Premium version.

I found bad entries, unchecked that key and then clicked remove bad keys. The RegVac has stopped working popup came up immediately. So I suppose it is some other key in Home Premium that is causing the problem.
Hopefully it is one key per version and not one [or multiple] keys per each individual computer.

   Vista does block certain areas of the registry.  When RegVac (or you, for that matter) tries to removes an entry from a blocked area, it can't be removed.  An error is not given in RegVac but RegVac does put the entry in a file called cantremove.txt and skips it in the future.
   I suspect this problem is a similar thing except that instead of Vista not allowing the entry to be removed, it throws up an error or closes down the program.  Hopefully, these restricted areas are limited.
   I would suggest narrowing down the error by removing all entries from a section of Classes Cleaner at a time and then narrowing that down even further to the offending entry.  Please post those offending entries here.  I will see what I can do about having RegVac skip them.