>It doesn't work on the other computers I assume because the ip changes
Not really (the ip change is another issue to be solved), because it does not change permanently and if you enter http://xxx.xxx.xxx.xxx
in a clients PC (where xxx.xxx.xxx.xxx is the current IP of the Xampp Server), you *should* have access to the Xampp pages. You *cannot* enter http://localhost
on another PC in order to reach the server, because localhost is always linked to the local PC.
Your Xampp server is part of your LAN (Local Area Network) and the LAN IP should be either like 192.168.xxx.xxx or like 10.xxx.xxx.xxx - this IP must be entered, if you are working in your LAN and if you want to reach Xampp from another PC (inside your LAN). If the LAN IP of the Xampp Server changes (due to DHCP configuration for example), you have to use the new LAN IP next time. Therefore it is a good idea, not to configure the Xampp Server to use DHCP, but assign a fixed LAN IP to it (for example 192.168.178.10 or similar).
If you want to reach the Xampp Server from "outside", you cannot use that LAN IP, as it is unknown outside (due to TCPIP Specifications). Only the Public IP of your router, which physically connects your LAN to the internet, is known outside. Therefore, you have to enter this routers public IP into the browser, if you like to reach your Xampp server. But: obviously this IP is *not* the IP of the Xampp server, but the IP of the router. And that is the part, which you still did not manage, you must configure a so called "port forwarding" in your router, all requests coming in on port 80 (which usually is the port of http / Xampp), must be "forwarded" to the local server, which hosts the Xampp package. If you cannot find a "port forwarding", it may also be called "virtual servers" or "exposing hosts" or similar. It might be a good idea to tell us, which router you have, if you cannot find anything like that; maybe your router does not support port forwarding (what would be the end of the mission!).
>I registered for no ip but don't really no what to do.
This is due to the problem of the IP change (which is caused by your provider, usually every 24 hours). You may tell someone your routers current IP and he may reach your server - but the next day, this IP "is gone" and does not work anymore, so you have to tell him again the new IP. This is uncomfortable. Therefore you can register for a fixed domain name, and in the configuration you have to assign your (current) routers IP to this domain. If you did that correctly, someone from outside can reach your Xampp server by entering http://blabla.no-ip.com
(where blabla.no-ip.com is your registered domain). But what happens, if your routers IP changes (again)? This is the most sophisticated part (and again, depends on your router): many routers do not only support port forwarding, but also support dynamic IP providers like "no-ip" or "dyndns" etc.. You should (once again) look for a menu or dialog in your routers configuration, where you can enter the registered domain name (from no-ip), the corresponding user name and password. The router will automatically update the no-ip configuration on IP change! Thats the trick. The problem is, your Xampp server does not recognize when the public IP of the router changes - but the router does! If your router does not support "dynamic dns" (it's mostly called like that), you may try to install a little program on the Xampp server, which configures the no-ip service every hour or similar, but as you see, this is not a very satisfying solution, as this is perfectly the job of the router.
>how will this effect the security of my computer
Not really. It is still protected by firefalls and (most important), it is still protected by NAT. You only open one (or two) ports for direct access and there is only one application (Apache) listening on that port.
>will this slow down my internet
Yes. And, most important: probably your Upload Speed (what is the Download Speed for the clients) is by far less than the Download Speed of your DSL Provider. A typical configuration is 16.000 kbit/s download speed but only 1.024 kbit/s upload speed (or less - 512 or 256 or 192 kbit/s are very popular also). For most projects this is the knock out.
>will the internet providers get mad due to people constantly downloading stuff.
No, i don't think so, as this "download" is running rather slow (see above).