You need new kernel drivers to handle the IO. Windows is going to act very stupid if you don't format clean. As a rule, its usually a good idea to have two drives (or at least 2 drive partitions) when dealing with any OS. One for the OS, and the other for your applications and data. Then if you want to upgrade, you only need to update the OS partition.... this works better on OS X than it does in Windows because Installshield and company tend to litter files all over your drive so you sometimes have to reinstall a lot of apps (OS X applications are one file 'bundles' so you don't have to deal with this or the registry), but keeping your data seperate from the OS is something you should aspire to.