You can download a trial version of Office 2010 from Micro$oft which would have powerpoint as part of it.

Failing that the only free alternative is OpenOffice. I say only as I havent searched for an alternative freebie.

Or lookie here http://www.presentationmagazine.com/...ownload-51.htm

Or failing that, ask work to provide you with it.

Hope this helps in some way