Any free full version of PowerPoint is illegal.
If you can not afford a legal copy of Microsoft Office use OpenOffice (free) instead. OpenOffice Impress is a clone of PowerPoint and can open PowerPoint files and save as a PowerPoint File.
Download a Microsoft Office 2007 60 day free trial
http://office.microsoft.com/en-us/products/HA101741481033.aspx
Trial programs contain the same functionality that you get when you buy the perpetual versions — but only for a limited time
2007 Microsoft Office release system requirements
http://office.microsoft.com/en-us/products/HA101668651033.aspx?pid=CL100796341033
Microsoft Office Suite comparison
http://office.microsoft.com/en-us/products/FX101635841033.aspx
http://office.microsoft.com/en-us/suites/FX101757671033.aspx
Microsoft Office 2007 Home and Student for 3 PCs $121.42
http://store.royaldiscount.com/miof20hoands1.html
Office Professional 2007 (licensed for up to 2 home computers)
Price $167.99
http://www.studentdiscounts.com/index.asp?PageAction=VIEWPROD&ProdID=14185
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OpenOffice (free)
http://www.openoffice.org/
Can import from Microsoft Office application and export to Microsoft Office application unless noted:
Base = Microsoft Access
Calc = Microsoft Excel
Draw = Similar to Microsoft Publisher but not directly compatible
Math = Create equations
Impress = Microsoft PowerPoint
Writer = Microsoft Word
No equivalent = Microsoft Office Office Outlook