For Deep Space imaging try
Deep Sky Stacker (freeware) or the latest version of
Images Plus v3 (not free).
Both are superb and I especially like IP.
Also something like Adobe Photoshop CS2 or better still CS3 or the cut down Adobe Elements v6 will give you great control in layering and finishing off the image.
There's also Paint Shop Pro XI but it hasn't got the full suite of features that Adobe offers.
Eventually you'll want to shoot your images in DSLR Raw format for a better result.