I have a model with a lot of vertices and I want to massivly decrease that amount but without effecting the model too much. Can anyone help me. 
there’s a decimator modifier, also a decimator script, and either a modifier or a script called ‘unsubsurf’ which I have never tried. However, the very best, and most time consuming way, is to use retopo to build a new mesh on top of your old mesh, using it as a form, and trying to optimize the topology as you go. decimator is pretty much instant, so you may wish to try that first and decide whether you like the results.
Thanks the decimate worked quite well but I have found a poly reducer script and was wondering if it would be more effective. And if so how do I put the script into Blender. Thanks again.
Open the script in blender’s text editor, then press alt+p to run the script.
The Decimate modifier destroy the UV map if you have one on your high poly model.
The Poly Reducer script tries to preserve the UV map (and weighting) from the high poly model, but works much slowly obviously.
To use the Poly Reducer, assuming you’re using 2.49b, go in Edit Mode then click on Mesh -> Scripts and the poly reducer is there :
Thanks benu and Modron that knowledge will come in handy when making my game and animating. Thanks to Robsoie aswell but just wondering when using the Poly Reducer it turns the faces into triangles. And I thought triangles were bad and you should try to avoid them when animating or using the game engine?