OEM stands for Original Equipment Manufacturer. In this context, it means a program supplied by someone other than the maker of the software, usually a disk that came with a new PC. These are often available in the aftermarket at much less cost than the full retail version of the program. If they have never been installed and registered, they are legal although the software companies are not particularly happy to lose full-price sales this way.

I also got my version of MS Word, the word processor this way. It is shipped in a package of software that many PC makers include with their product. Word costs several hundred; this package was less than $50. It qualifies for upgrades as well.