<p>I have a website (ASP.NET) and some winforms(.Net 2.0) for a project (written in C#). I use the webservice (IIS6) for task that both require like sending email inside the business. </p>

<p>I think Webservice is nice but I would like <strong>from your experience</strong> what should and what should not be in a webservice?</p>

Well it sounds like you have a limited Service Oriented Architecture (at least, that's what I think you're getting at), which according to Gartner means you'll be rich soon.  :)
I find that the benefit of SOA for me really comes down to the heterogeneity of the systems involved (sounds like yours doesn't qualify there because it's all .NET), and the negative of SOA is primarily because of the verbose nature of XML.  True, you don't
need
XML for SOA, but it's the current majority, IMHO.
But if you're not concerned about the bandwidth/parsing penalties, who cares?  Maybe you're not piping through 10,000 service calls a minute.  With this style of implementation, you're following
DRY
, just with a WS instead of a sub, and you're adhering to a standard that is by nature compatible with multiple systems.
There's worse approaches.