I see that for folders created under Win XP I have some user groups that do not exist for 'normal' folders (folders created by me recently under Windows 7).
For example, for the Win XP folder I have:
* Creator owner
* Account unknown(S-1-5-21 blablabla...
For Win7 folders I have:
* Authenticated users
Trying to delete the 'Account unknown' gives me:
Windows Security You can't remove Account Unknown(S-1-5-21-1220945662-706699826-839522115-1003) because this object is inheriting permissions from its parent. To remove Account Unknown(S-1-5-21-1220945662-706699826-839522115-1003), you must prevent this object from inheriting permissions. Turn off the option for inheriting permissions, and then try removing Account Unknown(S-1-5-21-1220945662-706699826-839522115-1003) again.
Now you will be able to access the XP folders like any other regular Win 7 folder.
I have found that some junk is left behind if you are using the first solution. More exactly you will see an account called 'Unknown user' appended to all yuour old folders. The best solution is to rename the current folder for example from 'MyData' to 'Old', make a new folder using the original name ('MyData') and move all content from the old folder ('Old') to the new folder ('MyData'). The new MyData folder will have the correct permissions applied to it.