I partitioned a 500GB HDD into two almost equal sized partitions. I installed Windows 7 Ultimate x64 SP1 on partition "C", then Windows XP Pro x86 SP3 on partition "D". I set them up to dual boot using EasyBCD, and all worked fine. The owner of the PC decided he wanted Windows XP Pro x64 installed in place of the x86 version (to be able to use all of the PC's memory), so I formatted partition "D" and started a clean install of Windows XP Pro x64. When the new x64 install reached the point where it automatically reboots to complete the installation, the PC failed to boot up to finish the install. When I tried to reboot the PC again, the EasyBCD boot menu was missing and I was unable to even get Win 7 to boot. After many attempted fixes to get Win 7 up and running again, I was finally able to get it running after using the Win 7 installation disc to get into the "Repair Windows" menu. I opened the Command Prompt, and executed the "BOOTREC.exe/FIXBOOT" command. After a reboot, Win 7 booted without a problem. What I would like to know is how I should go about reinstalling (or completing the Win XP x64 installation without compromising the working Win 7 OS? I'm sure it would be better to install the Win Xp OS on a second HDD, but the PC owner wanted both OS's on the same HDD.