OEM stands for Original Equipment Manufacturer like DELL or HP.
1. OEM Windows Vista is cheaper than full retail and around retail upgrade. 2. OEM edition does not include manual or box. 3. OEM is tied to the motherboard on the first install. 4. You can't contact Microsoft for support. Retail version includes 90 days free support.
#1: Microsoft Windows Vista Home Premium Retail $240 - OEM $120
You can get around #3 by calling Microsoft and telling them your motherboard has failed.
Note: 1. The software on disc is EXACTLY the same as retail. 2. OEM is tied only to the motherboard. 3. Both OEM and Retail give you the right to install only on one computer. 4. You can buy OEM at any store where they sell retail. 5. It is usually bundled with a token hardware due to Microsoft's rules that OEM should only be sold when a new computer is assembled.
What do we recommend? OEM.
Important: Website / email / eBay / anyone offering download edition of OEM is always illegal / pirated software. OEM edition comes with disc and Certificate of Authenticity.