So you can only dual boot on the same hard drive?
Not so - one of my systems dual boots XP, Vista, W7 and OpenSuse 9.3. I'm not clear on why you don't want to be able to see the XP Partition when you're in W7. There's always the option of not looking. However, you need to look to see if the root of the XP drive has a file called NTLDR and another called NTDETECT. If they don't show up, go to windows\repair and copy them from there to the root.