The folders or files or even an entire partition could be restricted in the previous instalation by 
any user, so now those restrictions keep in there for even the administrator. 

I've had that problem three days ago, I couldn't modify those restrictions to various folders. 

Win XP installs itself with the "Use simple file sharing" option in the explorer-->tools-->folder options-->view, menu. 
Just deselect it, and modify the properties of any folder, archive or partition you could not access:

1- right-click the object and select properties 
2- go to the security menu and then click the "Advanced" button. 
3- Make sure your user account stay in the "Owner" menu. Then click "Ok" and again "Ok". 
4- Repeat Step 1 
5- Go again to de "Security" menu and add your user account name in the "Group or User 
   names" field, and select "Full control" on the field below. 
6- Press "Ok" and that's all.