When Apache won't start the first thing to do is to read the \xampp\apache\logs\error.log file for clues.
You could use the version 3 control panel found in the xampp installation folder:xampp-control-3-beta.exe
Right click on it to select to run as an admin for best results.
Stop all XAMPP components and remove the ticks in the Svc check boxes then click on Exit in the older 2.5 version before starting this version 3 control panel.
Better still, the latest and greatest Control Panel Version 3
download post is linked below my signature.
Simply extract all the files from the CPv3 folder in to the xampp installation folder and overwrite when asked - do not extract the CPv3 folder complete, only extract it's content.
Then right click the xampp-control-3-beta10.exe file to run as administrator and to create a shortcut to the desktop for example, for easier access.
Make sure that you have exited all other control panel versions before starting this latest version.
This version 3 (XCPv3) control panel has many new features including a much more detailed log window and a button to easily access the log files to help with troubleshooting start and running issues.