Ben Armstrong: A while ago I made an open offer to people to contact me if they had experienced issues using Hyper-V backup.  A number of you contacted me with reports of large registry files in the parent partition – that would slow down system boot over time.

Well, after a lot of investigation we have figured out the cause and a hotfix is now available.  You can download it from here:

http://support.microsoft.com/kb/982210

I recommend that if you are using backup regularly on Hyper-V that you install this hotfix.

read full article at source