What software have you got?
Deep Sky Stacker is freeware I think and should do what you want with a minimum of fuss.
http://deepskystacker.free.fr/english/index.html
Registax should be able to handle it provided you only want to average the images and not sum them. I have stacked individual images in Registax before, but a realise there may be a warning message that comes up if your images are over a certain size... I'm sure it still worked OK for me, though.
If you have Photoshop, you can stack them manually and do a variety of different blending modes, but if you have PS CS3 it does the stacking, registering and blending so well and easily compared to the older version of PS. I'm very impressed with CS3!
Al.