See if my post here can get this resolved for you.
Link Removed
There is probably a more global way to do this, but I don't know it offhand. I am not in Win 7b at the moment to investigate.
EDIT: Try the following.
Ok, I only use Win 7 part-time (I boot 4 different OS's), and I did not remember exactly what I did to access my XP partition.
The following should work for you (you do not need to be in safe mode for Windows 7).
Open Windows Explorer in Windows 7, right-click on the XP partition, choose Properties, then the Security tab. Under Groups or User Names, you should see several listed. Look for Administrators and Users.
If you are missing either one of theses, click on Edit > Add, and type Administrators...