<asp:Button ID="btnAddFolder" runat="server" Height="24px" Text="Add Folder"
        Width="148px" onclick="btnAddFolder_Click" />
    <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
    <br />
    <asp:Button ID="btnRemoveFolder" runat="server" Text="Remove Folder" />
    <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
    <br />
    <br />
    <asp:TreeView ID="TreeView1" runat="server" ImageSet="XPFileExplorer"
        NodeIndent="15">
        <ParentNodeStyle Font-Bold="False" />
        <HoverNodeStyle Font-Underline="True" ForeColor="#6666AA" />
        <SelectedNodeStyle BackColor="#B5B5B5" Font-Underline="False"
            HorizontalPadding="0px" VerticalPadding="0px" />
        <NodeStyle Font-Names="Tahoma" Font-Size="8pt" ForeColor="Black"
            HorizontalPadding="2px" NodeSpacing="0px" VerticalPadding="2px" />
    </asp:TreeView>

string pathToCreate = "~/UserFolders/" + TextBox1.Text;
if(Directory.Exists(Server.MapPath(pathToCreate))
{
   //In here, start looping and modify the path to create to add a number
   //until you get the value needed
}

//Now you know it is ok, create it
Directory.CreateDirectory(Server.MapPath(pathToCreate));

int SHCreateDirectory(HWND hwnd, LPCWSTR pszPath);