How come Adobe Flash Player does not stay installed on my computer?

When I first open my computer (Windows Vista 32 bits) in the morning and reach, for example, a blog post that has a video in it, I get the message:

You do not have Adobe Flash Player installed. Click here to download.

I do and download the appication.

I'm okay for the day.

Next morning.. same scenario !

I do not understand why

Help!

Merci

Janice