What to check on the listing
- How many devices the licence covers, and for how long.
- The exact product name. Vendors sell several tiers under similar names, and a key for one tier will not activate another.
- Whether the key is for a new installation or a renewal — some vendors treat those differently.
Remove any other antivirus first. Two security suites on one machine will fight, and the resulting errors look like a bad key when they are not.