If you've got OS X 10.7 this should be easy:
- Plug the keyboard in
- Open System Preferences – Language and Text – Input Sources
- Behold, in the list of input sources, alpabetically below British, one called "British - PC"
- Choose it.
Done. What could be easier? If you keep the ordinary British one ticked as well (e.g. you have a laptop and second keyboard) then you'll want to tick the 'Show input menu in menu bar' option so you can easily switch between them. I like have keyboard viewer in the input list too, so I can see what layout the computer thinks I've got.
If you're on an older OS X and don't have British PC layout in your list, this post includes a link to download a British Windows Keyboard Layout for Mac.
The windows key should work as your command key, and control and alt should work as expected. If, in addition, you go to System Preferences - Keyboard - Keyboard, you should also see a button called Modifier Keys, which allows you to swap around the ctrl/command/alt keys.
I do wish Apple would make up their mind about calling the alt key 'option'. On my MacBook, the key is labelled 'alt' but in the Modifer Keys dialog box it's called 'option'