RegVac Registry Cleaner says the backup registry file can be found  in the Windows Directory under "system32/RegVac".

I've checked that path but can't find RegVac.  Where else could it be saved?
Thanks

   The registry backup is put in C:\Windows\system32\RegVac in a folder named with the date, for example, 20080505.  This is assuming that your Windows folder is C:\Windows.  If that is not where your Windows folder is, please look in the Windows folder.  RegVac does not put it anywhere else.

Okay, I found it! I don't know why I couldn't find it yesterday.  I thought I went to the same location.  

A follow-up question:  When RegVac backups the registry the backup file is approximately 50MB in size but when I back up the registry manually I get a figure of about 120MB.  Why the difference?

   The registry backup file is a zip file.