class DirectoryActions
    {
        string userName, password, domain = "";          
        public DirectoryActions(string username, string password, string domain)
        {
            this.userName = username;
            this.password = password;
            this.domain = domain;
        }           private DirectoryEntry GetDirectoryObject()
        {
            DirectoryEntry oDE;
            oDE = new DirectoryEntry("LDAP://" + domain, userName, password, AuthenticationTypes.Secure);
            return oDE;
        }            

        public  DirectoryEntry GetUser(string mail)
        {
            ArrayList mylist = new ArrayList();
            DirectoryEntry de = GetDirectoryObject();
            DirectorySearcher srch = new DirectorySearcher(de);
            srch.Filter = "(&(objectClass=user)(mail=" + mail + "))";
            srch.SearchScope = SearchScope.Subtree;             SearchResult results = srch.FindOne();
            DirectoryEntry d = results.GetDirectoryEntry();             return d;
        }
    }

After you install Windows SharePoint Services 3.0 SP1, declarative workflows will not start automatically if the following conditions are true:

1. The Windows SharePoint Services Web application runs under a user's domain account.
2. The user logs in by using this domain account.
3. The site displays the user name as System Account.

Here is the link to the solution :  http://kbalertz.com/947284/declarative-workflow-start-automatically-after-install-Windows-SharePoint-Services-Service.aspx  

Most of people wants to know differences between WSS 3.0 and MOSS 2007. You can find the comparison sheet via this link. 

void GetSites(string URLOfWebApplication) 
{
   SPSite SiteCollection = new SPSite(URLOfWebApplication); 
   SPWebCollection Site = SiteCollection.AllWebs;

   foreach (SPWeb x in Site)
   {
      SiteListComboBox.Items.Add(x.ToString()); 
   }

   SiteListComboBox.Text = "Please Select a Site";
}

void GetItemsFromDocumentLibrary(string URLOfWebApplicationAndSite, stringDocumentLibrary)
{
   SPSite mysite = new SPSite(URLOfWebApplicationAndSite);
   SPWeb myweb = mysite.OpenWeb();
   SPFolder mylibrary = myweb.Folders[DocumentLibrary];
   SPFileCollection files = mylibrary.Files;

   foreach (SPFile x in files) 
   { 
      ItemListBox.Items.Add(x.Name.ToString()); 
   }
}

SPSite site = new SPSite(" http://litwareportal:28779" ); 
SPWeb web = site.AllWebs[0]; 
SPUser user = web.AllUsers["CONTOSO\\semih"]; 
SPGroup gr = web.Groups["Home Visitors"]; 
gr.AddUser(user);