Hello,
I am writing a program in c# that should convert postcodes to the corresponding coordinates.
I assume there should be a sort of database,maybe gps coordinates data base, but i do not know how to access them. On the other hand i see that there are some websites that when you enter the postcode in the related text box, it will give u the associated coordinates. is there anyway that i make sort of query from my application to those websites,(sth like this: when a user enters a postcode in my windows application, the postcode field in that website be filled n gives me the coordinate), or any other suggestion to convert postcode to coordinates?

Many thanks,

Here are a few links:

Specific to what you were looking for:
http://www.stevetrefethen.com/blog/UsingGoogleMapsforGeocodinginC.aspx

Google's reference for geocoding
http://code.google.com/apis/maps/documentation/geocoding/

How to get an account and use the RESTful services
http://code.google.com/apis/maps/documentation/mapsdata/developers_guide_protocol.html

I've used a similar API with Netflix but I haven't used the Google one in particular. Those links should get you started and give you some other search terms to try.