Hi,
a 'custom' install is basically a fresh or clean install and will wipe the C drive only. If your running 32bit os and want to change to 64bit then you'll need to perform a custom install as opposed to a 'upgrade' install. An upgrade install is where you just pop the install DVD in the drive and follow the prompts. To perform a custom install you'll need to enter the bios and change the boot order so you boot off the windows media whether it be a DVD or thumb drive. That means changing the boot order so that either the DVD drive or thumb drive, whichever holds the os, is first.