|You installed XP and then Windows 7 into the same partition?|
In this case it is to be expected that XP is inaccessible since it no longer exists. Two operating systems cannot share a partition.
Files encrypted with one installation of Windows will be inaccessible with another, even if the user name and password are the same. Unless you took the proper precautions, such as backing up the encryption certificate, these files will be unrecoverable except by advanced and very time consuming methods.
The best way to dual boot XP and Win 7 is to install XP first and create a separate partition for Windows 7 and install it there.