The boot drive is usually in the sata connection number '0' or if no number '0' then number '1'. They should be marked on the motherboard.

This might help, click here

Nothing to do with the drive

Its to do with access permissions, the old op system on the sata has different account than the XP therefore XP will not grant access.

Access Files If you get Access Denied in WinXP Pro. Try “taking ownership” of the file/folder before it can be accessed. First of all go to Folder Options, View Tab and in the Advanced Settings at the end, untick “Use simple file sharing”, which will allow the Security Tab to be shown in the file’s properties. Also, in Folder options, make sure hidden and system files can be viewed. To take ownership of the file (it’s best done at the folder level), right-click it, Properties, Security Tab, add your user name, fill in all the permissions and Apply. You can now access it. http://support.microsoft.com/kb/308421/en-us

Access Files XP Home Simple file sharing is enabled by default in XP Home Note You must be logged on to the computer with an account that has administrative credentials. If you are running Microsoft Windows XP Home Edition, you must start the computer in safe mode, and then log on with an account that has Administrative rights to have access to the Security tab. or http://www.whitsoftdev.com/rshx/ or http://www.mydigitallife.info/2006/08/03/enable-and-display-security-tab-in-windows-xp-home-edition/ and click on the link at part one and download the required program. Follow the instructions and the security tab will be enabled. PLEASE READ CAREFULLY OR YOU COULD DENY ACCESS TO ALL YOUR FILES.

http://support.microsoft.com/kb/810881