Most computers now come with Flash pre-installed... so no problem for new machines. Flash was not installed on my computer when I visited my first Flash site... but I was then offered to download the player. Now, I don't know if that offering was due to the flash programming or my machine just not finding a suitable player for that file type. A lot of sites present a short flash animation and ask the viewer if he sees it. If he does it means Flash is installed. If he doesn't, the site can redirect him to a Flash download site or offer him an alternate html straight version tour!
;-)