There are two types of OEM licence available from Microsoft, (or should say
Were available),
There are those that
Snick has described accurately above, these are the ones that often appear on sales sites online that have been modified to fake the SLIC tables in software, (which Microsoft is aware of and quickly blacklist the keys), they also usually contain other additions such as keyloggers, Bitcoin miners Etc so should be avoided at all costs.
Microsoft also developed another type of OEM licence, this was aimed at Small System Builders, the Small "Mom & Pop" electronics shops, and some say personal users (there is some "discussion" about this one, But I can say that I've never heard of Microsoft ever not honouring one of these licences because it was purchased by an individual, rather than a system builder
Now there are still some of these System Builders OEM Disk and Licence packs available, they are not easy to find and it may be that one of the small Mom & Pop's local to you may have one on a shelf somewhere. If you are trying to obtain a legit copy then I would say you need to look at the large online IT stores, (not general store types Like amazon or Ebay), and be prepared to pay a premium price now that legit stock is very rare. Last time I saw a price it was around $150 but I would not be surprised if this was now nearer $200.
Although the licence should only be acquired in a sealed pack where the disk and key are not visible without breaking the seal, you do not need to lse all your programs and data, see the Inplace upgrade tutorial on this site
