Surprised (but glad) you were allowed to just 'take it back' -- is this a laid-back Dutch thing or did you have to threaten them?
My recent switchers' perspective goes like this (after you get it up and running as per Simon's post).
Get a copy of Parallels because you WILL miss windows-only apps. It's not expensive (50 quid?) and it works absolutely flawlessly and very fast. It's possible, should you wish, to forget you are actually running Windows in a virtual machine on a Mac. (Strictly speaking you're not, entirely, since it will use VTx, i.e. native Intel instruction processing--if that's the right words--rather than emulation, for much of what it does).
Get a copy of OpenOffice.
Get a copy of the latest Lightroom beta.
The rest is getting used to the quirks.
The weird mouse acceleration profile may do your head in. I fought it for a while and looked for gizmos to work around it (of which there are several). It seems I'm not alone in finding it very annoying but exclusively in people moving from Windows so it's not that Apple's is 'wrong', it's just that they are different. I have now almost got used to it but I'm still not as quick mousing as I am on Windows.
You will learn quickly with much swearing that Shift-End doesn't work any more to select to the end of a line of text. You have to start using Apple-Home. PgUp and PgDn work differently as well. You will get a large bump on your forehead.
Also note that Ctrl-C becomes Apple-C, which is all fine and dandy except that Apple is where Alt is on the PC keyboard, not Ctrl. You can either remap the keys on the Mac, or just get used to it. I've done the latter, and find that once I've used one or the other machine for about 30 seconds, I subconsciously switch into "OK, I'm on THIS machine which uses THAT position for 'Ctrl'" mode, and it's not an issue.
Same applies to loads of other quirks... in general I've found it easier to go with them than fight them.
Prepare to be annoyed at the inability to do anything to files which are part of a set being copied from another place until the copy operation is finished.
Prepare to LOSE files if you try and copy a folder onto another folder of the same name. Windows merges... OSX replaces.
Blimey, that wasn't meant to turn into a novel.