Use a hard drive imaging/backup utility - Acronis is among the best, if not *the* best - and copy your existing installation over to the spare hard drive. Test it by unplugging the original HDD, and booting/using the computer from the "new" HDD for a while. Then upgrade the new install to 7. Once that's all done, plug your original hard drive back in. If it's not automatically set up, you can boot to your Win 7 CD and use the repair option to fix the boot manager - It'sll tell you there's something wrong and offer to fix it - Say yes, and you should be presented with the option to select which one on next startup.