I converted your email into a support forum topic.
Slate requires a 64-bit version of the Python runtime environment because it manages files that are too big for the 32-bit version. It is incompatible to have the 32-bit and 64-bit versions installed on the same system. So, the installer checks and halts with this message on a machine with this incompatible environment.
The good news is, there are only a few edge case instances where changing a 32-bit to a 64-bit version will not work. You have a 32-bit version running on a 64-bit Windows host (I know because our installer would have failed with a different error before this error if Windows were 32-bit). So, it is very likely your existing Python tools will run fine, and possibly faster because they'll bypass Windows' WOW system.
Most likely, you can simply use the control panel to uninstall the current version, delete the old C:\Python27 folder tree, then run the Slate installer which will install the 64-bit version.This article tells you how to re-install the 64-bit versions of any additional (non-standard) Python libraries. If you Follow the instructions in this link, it helps you make a list of additional Python libraries that are in your current installation and help you re-install them.
The edge case would be if you have additional libraries that don't have a 64-bit version. Python itself won't run the 32-bit library on a 64-bit runtime. In this case, you'll have to choose one or the other. You can check for library features here: http://pypi.python.org/.
I hope this gets you through the installation.