Well, the number of fonts that are installed certainly has a bearing on the speed that Photoshop initialises, and it may well have an effect when you get the application up and running, too - as well as on other applications that use fonts such as MS Word.
You can manage your fonts with the Fonts control panel applet, but to be honest, that's a bit crap. What you really need is a 3rd-party utility such as The Font Thing. It's easy to use and above all, it's FREE!
The link is here:
Sue Fisher: The Font Thing