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
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:
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 :-/