The trouble with OEM (that is it does not come with the computer) is that your Monitor and Sound may not have the correct software. However, if you can identify them, they are usually free to download from the Net!
I have both the original OS and an OEM because I did not know how to delete the partitions installed with the Media Center, which was the original. But I did find the appropriate software.
I have now reverted to the original OS (XP) and waved goodbye to the £80 the OEM cost me!