Hi,
A lot of fish are prone to whitespot (also known as ich) when they are moved from one place to another, which is why a lot of fish when first introduced to a tank develop it. It's also common when water conditions are poor. It is, however, highly treatable when caught early. When you get your ich treatment (as caz27 says interpet anti whitespot plus will do the job, I have also used Protozin and found it to be very very good and quite gentle), make sure you don't overdose. What other fish do you have in your tank? If you have any scaleless fish (e.g. loaches), you will have to halve the recommended dosage that it says on the bottle. Also, contrary to what it says on the bottle, you need to continue treating the fish every day until there isn't a single whitespot left on
any fish. This is important as it's only when the spots have dropped off and the parasite is in its free swimming stage that the infestation can be treated.
One more thing that helps - if you increase the temperature in your tank to 80ºF this will speed up the lifecycle of the parasite and hopefully kill it off quicker.
If you go to the top of the beginners forum you will find some great articles on ich and fish disease in general, hope it helps!
Now the only thing left to figure out is why your fishies got ill in the first place. Is it a new tank? If so, you might want to read the article called 'avoiding new tank syndrome', it's really good.