When I first started using VMware Fusion, before v1 was released, on my MacBook Pro which doesn't have a Print Scrn key I wrote my own program that sat in the system tray by the clock on the Windows Taskbar to handle that function. There are also lots of third party utilities that do that and more.
Somewhere along the way VMware Fusion added a Print Scrn Send Key command to the Virtual Machine menu.
One can also use the virtual keyboard in Windows. Have a look at: Type without using the keyboard (On-Screen Keyboard)
Additionally one can also use the various OS X side Screen Shot commands.
Built-in Mac Screen Shots Commands
Key Combination - Result
Command+Shift+3 - Capture entire screen and save as a file.
Command+Control+Shift+3 - Capture entire screen and copy to the clipboard.
Command+Shift+4 - Capture dragged area and save as a file.
Command+Control+Shift+4 - Capture dragged area and copy to the clipboard.
Command+Shift+4 then Space bar - Capture a window, menu, desktop icon, or the menu bar and save as a file.
Command+Control+Shift+4 then Space bar - Capture a window, menu, desktop icon, or the menu bar and copy to the clipboard.