The program that opens when you double click on a file is determined by File Types (Under My Computer - Tools - Folder Options - File Types tab).
Because you don't have Powerpoint installed There is unlikely to be any entry for the file extension .ppt. (If you scroll down the list to find the .ppt entry in File Types it's likely to be missing.) You can make an association with Open Office Impact by using the "New" button entering PPT and associating it with Impact using browse.
Or try and find the ppt file using Windows Explorer, right-click on the file select "Open with" select OpenOffice Impact but remember to tick the "Always Use this program box.
You can also download a free PowerPoint viewer that allows you to open and view but not create or edit presentations. Office 2007 version available click here