Where and when do these notices pop up? Does it only happen when you are in Safari / Firefox? Only on certain websites? They could be fakes that are often used to try and trick folks into downloading malware. Could you possibly provide a screenshot? Fakes, while similar, can usually be spotted by those who have seen them before.
To follow on from Pr0digy V.
When you do get an update notification do not use the link it contains. Go to the Adobe website and download the latest player from there. Flash is becoming an increasingly more common way of getting people to download malware (as well as helping the delivery of malware after it is installed)