this is a brand new problem issue for me...
A new hp laptop running Vista 64 bit Home Premium.  had a lot of problems getting it to allow Outlook 2007 to install correctly....  several removal and reinstalls later I decided to run RegVac....  in 2 1/2 hours RegVac found 22979 Bad Link Entries, worked another 2 1/2 hours and listed 11393 items deleted to the Undo List.  I was watching RegVac.exe on the Task Manager the whole time.  I "Finished" and the program closed properly.

Now when I try to start RegVac, the screen starts with the copyright frame, then goes all black, then when i click on it, it changes to a white rectangular void in the center of the screen and RegVac is "not responding" on the Applications tab.  In Task Manager 'Processes' tab, RegVac has been "processing" now for 2 hours+ at about 50% processor capacity, but still no welcome screen...

Does anyone else have this experience??  is RegVac hung??  or corrupted somehow??
Anyone??

   I have not heard of that.
   The white rectangular thing sounds like Vista is not drawing it all.  Can you drag the RegVac form offscreen?  If so, try dragging it offscreen, drop it, and then drag it back onscreen.  Hopefully, that will force Vista to redraw it.
   The other possibility is that it is hung for some reason.  First I would make sure that there is not an error message being displayed under RegVac's form (try selecting it, then selecting a different program, and then selecting RegVac again, or try using Alt-Tab to cycle away and then back to RegVac, or try moving RegVac to see if anything is behind it).
   If that does not work, try renaming the regvac.ini file and the current undo file in the RegVac directory.  Then start RegVac and see if that corrects the problem.  Maybe the undo file is too big.  Renaming it should fix that.
   I doubt that RegVac is corrupted, so reinstalling RegVac should not make any difference.

Thanks Ray...
Actually this happend to me twice on the Vista 64 OS.  The first time was following a very lengthy scan that i cancelled because it was running so long... over 9000 items.  Later restarting RegVac, the Welcome Page did not appear, only the copyright box frame, no printing, just the shape of the frame like a rectangular hole in the middle of the screen in front of all other windows.  the 'hole' could not be moved, and could not get RegVac to open.  So I tried uninstalling RegVac and received and error message regarding the uninstalltion.  did not save the message and Regvac was inoperable. Then I recovered the use RegvVac by using a previously set Restore Point (this action eliminated the last too large Undo file).  A few days later I ran RegVac again, this time in Novice Mode, it collected some 16oo+ items to delete.  All was good.  then a few days later i asked it to run in Expert Zone, checked the 'BadLink' - 'Entire Registry'  and the 'Remove Suspect Keys' boxes, and kept track of the activity on Task Manager.  it ran for 5 hours, collected  22969 items to delete.  the Undo file was over 15Mb...  This time I could not get Regvac to restart again either, as you suggest the Undo file is probably way too large.  Since I had other issues with Outlook 2007 to fix, I reset again to and earlier "Restore Point".   Right now all is well with Regvac, it will load, but I am nervous about running it, having read the earlier issue concerning Vista 64 and 'Windows Defender' and 'Windows Media Network' services conflicts in V 64.  I do want to find out more about that to make RegVac safe to use on my Vista 64 OS...  I know I need a good cleaning because of a lot of intalling and uninstalling to get all the different software working together...  sorry this is so windy...
I look forward to other develpment here on this issue, and will help anyway I can..  Please advise, and comment...  thank you very much...  My Regards!
PS:  how large an Undo file can RegVac handle??

I don't know how large of an Undo file it can handle.  It has to do with the fact that in programming all number types have size limitations.  I looked at the code and did not see any small number types.  I also looked in Windows documentation for limits but did not find an answer.