Let me elaborate on my comment:
1. Is Apache only listening to 127.0.0.1?
You could test this by typing the following in the command line:
>netstat -a -b -n
Find the line with httpd.exe:
TCP 0.0.0.0:80 WINDOWS-PC:0 LISTENING
[httpd.exe]
If the address is 0.0.0.0, then apache is listening on all interfaces. If it's 127.0.0.1, then you'll have to configure it to listen on all interfaces. To do this on XAMPP:
Go to your XAMPP folder's Apache configuration folder (XAMPP\apache\conf).
Edit httpd.conf's Listen line to be Listen 80
Restart XAMPP (Apache specifically)
2. Or are you behind a NAT?
Doubt this could be it because, even if you are behind a NAT, you should get a connection to Apache from your own computer. But still worth checking out.
Go to command line (Start -> Run -> cmd) and type the following:
>ipconfig
If your modem's IP is either 192.168.x.x, 172.x.x.x or 10.x.x.x; you're behind a NAT and would have to configure the router to do IP forwarding. This might be difficult/impossible if you don't control you're router.
3. Or, you're ISP is blocking port 80:
Finally, if you're not behind NAT and you're listening on all interfaces, baring some firewall misconfiguration, odds are your ISP is blocking port 80. You're simply SOL :-/