With the release of windows7, I opted to fresh install the windows7 64bit OS at my home PC. The installation was smooth and within 30 minutes I was running 64bit Windows7. I installed other important software for my regular activities and all ran fine with only exception that 32 bit software entries were not displayed in the context menus.
I later tried to see whether 64bit and 32bit OS can co-exist or not as dual boot. Since I already had the Vista(32bit) SP1 with me, I fresh installed it as dual boot on separate disk, hoping it should install fine without any issue.
To my surprise, I didn’t faced any issues while install. You will see the upgrade option as grayed out, because the Windows7 version is higher than the Vista version. Also you can’t upgrade Window7 64bit to Vista 32 bit. Overall, the installation went smoothly and after install I got Windows Vista as default OS in boot menu. This you can change back to Windows 7 using BCEdit tool.
There was no problem after installation of both 64 bit and 32bit OS as dual boot. Also order of installation (i.e. first windows7-64bit and then windows vista-32bit) also didn’t created any issue, as recommendation is to always install the lower version OS first and then higher version.