The primary reason is that by default, virtualization tools rely on a hard drive image to contain your Windows installation and files. The image appears to Windows like a normal hard drive, but on your Mac it appears as a single image file. This can make it difficult to directly access those files from Leopard.
Fortunately, the virtualization tools let you configure shared folders so that a folder on your Mac's hard drive is seen as a mapped drive in Windows. This isn't always the most convenient solution, but it is effective, particularly if you enable your entire Mac OS X home directory (or any commonly used folder) as a shared folder.
Note, however, that sharing your entire home directory with Windows has security implications, as detailed on the next page.
Both Parallels Desktop and VMware Fusion also let you drag and drop files to copy them between the Windows hard drive image and locations on the Mac's hard drive.
The most recent versions of Parallels Desktop and VMware Fusion include a couple of helpful features beyond shared folders. Both tools offer the ability to use a Boot Camp partition instead of a hard drive image file, which can then be accessed as described above. This means you can easily make use of virtualization with an existing Windows installation that was made using Boot Camp.
In the latest version of Parallels, you can launch a file browser from Leopard that lets you navigate and manage the contents of a virtual machine's hard drive image even if the virtual machine is shut down.
Finally, both tools support a windowless mode (known as Coherence in Parallels and Unity in VMware) in which Windows applications are displayed in the Dock alongside Mac applications. This mode also allows access to files from Mac or Windows applications regardless of whether those files reside within the virtual machine's disk image file or within the Mac's file system.
Cloning an existing PC
If you're moving from a PC to a Mac and planning to use either virtualization or Boot Camp to run Windows, you can skip most of the Windows setup process by cloning your PC's existing files and configuration.
Both Parallels Desktop and VMware Fusion include tools for migrating or cloning an existing PC to an image. If you're opting for Boot Camp, check out Laplink's PCMover utility, which allows very granular choices about how the file system and individual files and applications are migrated.