After seeing several threads relating to this issue, I decided to make this it's own thread instead of trying to reply to each. This article NOW resolves all issues pertaining to using Windows under Boot Camp through Parallels, INCLUDING activation.
Hopefully Parallels will be able to integrate these fixes into a future release.
Before doing anything, I would recommend preparing your existing Windows installation under Boot Camp, for Parallels, before trying to access it through Parallels. If you have already installed Parallels and now have problems, don't worry, this article should address and resolve these issues as well.
Something to know upfront: using your existing Boot Camp installation with Parallels will most likely require you to activate your Windows installation a second time.
The reasons for this are addressed in Section 4 of this article.
Section 1: Preparing your Boot Camp Installation for Parallels
(If you already tried to access your Boot Camp installation through Parallels, and can no longer get into Windows through Boot Camp or Parallels, please skip this step and come back to it after you have completed Section 2.
First we need setup one Hardware profile for Boot Camp and one for Parallels. (Credit for this fix goes to logan_barnett
1. Open your "System Properties" either through the Control Panel or by right-clicking on "My Computer" and going to Properties.
2. Go to the "Hardware" tab and click on "Hardware Profiles".
3. Rename the current profile "Boot Camp", and then copy it. Rename the copy "Parallels".
4. Under "Hardware profiles selection", make sure "Wait until I select a hardware profile" is selected.
5. Check the properties of each profile and make sure the following options are selected:
"This is a portable computer"
"The docking state is unknown"
"Always include this profile as an option when Windows starts"
6. Click "OK" on everything.
This will allow you to choose the correct hardware profile when booting into Windows using Boot Camp or Parallels. Select the "Parallels" hardware profile when using Parallels and select the "Boot Camp" hardware profile when using Boot Camp.
Click to expand...