Tom:
If you're referring to Ad-aware (by Lavasoft) and Spybot - Search and
Destroy (by Patrick Kolla) then you should know that both these tools are
excellent at what they do, and both are very safe to use on Windows. Yes,
they both do the same thing: search for and try to remove adware, spyware
and other malicious pests. However, these programs use different approaches,
so sometimes one will find something that the other one missed. This is not
the fault of the software designers; you have to understand that malicious
software is not a standard product. For that reason, many of us use both
products.
And these two are not the only ad/spyware removers, although they are quite
well-known. There are many other excellent programs that perform this
function. Choose the one(s) whose interface and feature set appeals to you.
And be sure that you download the product from the developer's web site.
Rocky