The most likely things are:
You have other programs/services running on Ports 80 or 443
Use the 'netstat' button in the Control Panel to see what is running on either ports 80 and/or 443. (you should use the xampp-control-3-beta OR download the newest version from the topic above in this board)
B: Your firewall may be blocking Apache - create an exception for httpd.exe in \xampp\apache\bin.