Why does the game keep telling me insert the game disk?

It thinks your disk is NOT an original: i.e.you're a pirated copy.

Well, the game's copy protection thinks the Need For Speed High Stakes disc you inserted is not original. This means either that:

1) Your cd drive can't co-operate with the copy protection system. In plain english, your cd drive doesn't let the copy protection system perform the necessary actions to determine whether your disc is original or not. Download a crack (or "fix" if you don't like the c word) or a NoCD patch to make the game start without checking if the disc is original ot not. You know where to find (torrents etc)

2)Your game disc is pirated. Go buy the game.

Install nfshspatch4_50 and add -patchrestart at the end of target path in your nfsHS shortcut properties. You may also go to www.iplounge.net/ -- the best non official support for HS.