If you've followed all the steps of Ashley's LAN preview guide and are still experiencing trouble under Windows XP, then this tutorial is for you. NOTE: Make sure to follow Ashley's Lan preview guide first and make all of those changes under Construct, although if your router is uPnP compatible, you might not need to do any manual port forwarding.
First things first, before attempting to manually open up the necessary TCP port range in your router configuration, I would suggest that you first follow these steps, as it is possible that if your router is UPnP compatible, it may not even be necessary to make any changes to the router itself.
However, if after these steps you are still unable to preview on LAN, you are going to need to open up the usual port range in your router settings from TCP port 50000 to 50100. This is usually located in the Port Forwarding / Port Triggering section of your router configuration page. Make sure to forward them to the IP address of your machine. As before, see Ashley's Lan Preview tutorial for implementing this.
Let's assume for a moment that traffic is being correctly forwarded to your Construct 2 IP address, the problem is that the Windows XP firewall, which is ON by default, is still going to restrict traffic, so you're going to need to create a firewall exception. In order to do this, go to your Network Connections, right-click on your primary connection (probably the wireless option for most people) and click Properties.
1. In the Properties window, click on the Advanced Tab, and then click Settings under the Windows Firewall.
2. Once again, click the Advanced Tab, and under the Network Connection Settings box, highlight the same network connection as before and click Settings.
3. Under the Services tab, click Add…
4. Type in 'Construct 2 Server' for the description
5. Under the name or IP address, I recommend typing in either localhost or the machine name, rather than typing in an IP address which may change if you don't have a Static IP assigned from the router. If you type 'localhost' Windows should just automagically rename it to the name of the System.
6. Type in 50000 for both the External and Internal ports and make sure TCP is selected.
7. Click OK and make sure that the Service is checked under the Services.
8. Keep selecting OK until you've closed down all the windows.
If you've followed all these steps, you should be able to use the Construct 2 LAN preview option normally, as the Windows XP firewall is no longer blocking it.
Hope you found this guide useful, and let me know in the comments or PM me if you have any questions.