The OEM and retail versions are the same except for the licence. The OEM version of Windows 7 can not be purchased seperatly. It is only sold with a system. It can however be install as many times as you wish. But remember you need activate windows each time you do a clean install. For this reason its best to have a backup image of your whole system using a program like acronis true image.
If you need help installing windows then check out this guide : how to install windows 7