Using a mac keyboard with a PC...

I'm not sure that you can. However you could use a tool like SharpKeys or AutoHotKey to map it for you (a key F13 thru F19 might be a good choice).

 

On my keyboard F15 works as pause.

 

I had this same problem when I was running very long code in Excel in Bootcamp, and I longed for a Pause/Break key. Needless to say, Bootcamp doesn't map Pause/Break to a MBP keyboard. Nor does my MBP have an F15 key. My solution was splashtech suggested, download AutoHotKey. Once downloaded open AutoHotKey in Notepad (if you can't open it with Notepad find the file in My Computer, Recently Modified Files, etc. and drag it into an open empty Notepad doc). As you can see from their default scripts the key codes before :: correspond to the keys you press to get the result following the ::. Under the 2 scripts they have as default I entered ^F12::^CtrlBreak. ^ being Ctrl and F12... So now I enter Ctrl + F12 for Ctrl Break on any other computer. You can make the hotkey whatever you please. A list of keys can be found here: http://www.autohotkey.com/docs/KeyList.htm.

If you're worried about running 3rd party software on you're beloved Mac while in Windows (as I was), this code is open source and virus free. Hope this helps some of you.

 

Mac Keyboard on PC

I spent ages trying to figure out how to do print screen on a Mac keyboard attached to a pc. Using the On Screen Keyboard should solve all keyboard problems.
I'm using Vista it might be slighty different on other OS.To access On Screen Keyboard go to:
All Programs
Accessories
Ease of Access (Might be Accessibility on XP)
On Screen Keyboard
Once keyboard is open click Keyboard tab and choose enhanced settings.
Click shift key to show uppercase keys.
To print screen. Make sure the On screen keyboard is as as off screen as possible (with just the top row showing) as it will be printed. Press psc.