Hi Waghmare,

Please refer below code and you need to add reference for the Microsoft.VisualBasic and use below article for modal popup and modify according to your need.

Open (Show) BootStrap Modal Popup from Server Side in ASP.Net

Code

Microsoft.VisualBasic.Devices.ComputerInfo OS = new Microsoft.VisualBasic.Devices.ComputerInfo();
System.Web.HttpBrowserCapabilities browser = Request.Browser;
StringBuilder sb = new StringBuilder();
sb.Append("OS FullName : <b>" + OS.OSFullName + "</b><br/>");
sb.Append("OS Version : <b>" + OS.OSVersion + "</b><br/>");
sb.Append("Browser Type : <b>" + browser.Type + "</b><br/>");
sb.Append("Browser Name : <b>" + browser.Browser + "</b><br/>");
sb.Append("Browser Version : <b>" + browser.Version + "</b><br/>");
Response.Write(sb.ToString());

OutPut

IE

OS FullName : Microsoft Windows 7 Ultimate

OS Version : 6.1.7600.0

Browser Type : IE

Browser Name : IE

Browser Version : 8.0

Chrome

OS FullName : Microsoft Windows 7 Ultimate
OS Version : 6.1.7600.0
Browser Type : Chrome51
Browser Name : Chrome
Browser Version : 51.0

Mozilla

OS FullName : Microsoft Windows 7 Ultimate

OS Version : 6.1.7600.0

Browser Type : Firefox48

Browser Name : Firefox

Browser Version : 48.0