buy powerpoint is the only way to get a key for it. or download the ppt viewer MS offers. it will let you view the file but not edit it. there are plenty of free office solutions out there if you don't want to buy MS office. they do the same things for (legally) free
It would appear that your computer came with a trial version of Microsoft Office (to include PowerPoint). That trial period has expired and if you wish to continue to use Microsoft PowerPoint, you will be required to purchase a license and obtain a Product Key. However, and as the others have already stated, there are other options for opening a presentation, such as OpenOffice.