You can View the Article here : http://msdn.microsoft.com/en-us/library/ms916810.aspx But the articles is for sharepoint 2003.
There is one more good Article in "toolbox for IT" which i found and seems to be a derived into much simpler form from MSDN Article.
You can view the article here : http://it.toolbox.com/blogs/sharepoint-blog/creating-a-custom-web-service-for-sharepoint-13553
But The above articles are for sharepoint 2003 . Its almost the same for 2007 but you have to make some changes to accomodate it in 2007 .
Here are 2 things you have to take care:
1. location to drop dll file: there is no \ISAPI\BIN in MOSS2007 (at least for now) put the dll in ASP.NET vitrual directory e.g. C:\Inetpub\wwroot\wss\VirtualDirectories\(sharepoint port:80)\_app_bin\mydllname.dll
2.Also make sure inside your .asmx file you have named your dll e.g.
[%@ WebService Language="c#" Class="mypackage.myclass,mydllname" %]
( Replace [ ] code on top with < > code)
Rest is all the same , and should work fine for you .
