The bootmgr error you're getting might be because your HDD boot priority in BIOS is set to the HDD XP was installed on. If you go in to BIOS, and switch it so that the Windows 7 HDD has priority, it should boot straight in to it without errors.
In Windows 7, open start menu and type "msconfig" and launch it, click the "boot" tab, and it will list OSs that are currently installed. Make sure Windows 7 is set to the default one. You can also delete the Windows XP entry if it's listed, too and if you're not planning on using XP.