Actually it sounds more like PHP script not working, but that is a different matter.
Provided you have not tinkered with the PHP script, you should be able to reinstall fairly easily.
I am working on the presumption you installed XAMPP in the root of C:\
first, backup your current XAMPP 1.7.4 installation. (this could be as simple as copying the entire XAMMP folder to a different location). If you are using MySQL, It would really be better to use phpMyADMin to backup the MySQL database, BUT if you are going to put XAMPP back into c:\xampp - it should be OK to use the 'copy/backup' method.
second un-register any of the components running as services. (Apache, MySQL, FileZilla, etc.) Unmarking the Svc box in the XAMPP Control Panel should do this.
reboot the machine and maje sure none of the services start.
now Delele (or uninstall) the current XAMPP folder.
Install a clean copy of XAMPP 1.7.3 into the same location your original installation was, get XAMPP 1.7.3 from the SourceForge Archivehttp://sourceforge.net/projects/xampp/f ... 20Windows/
make sure it works - nothing more.
MAKE A BACKUP!
(there's a free Windows backup program that works quite well, I use it to backup my XAMMP projects daily.)http://www.fbackup.com/
Now copy the files and folders that are needed/modified from your backed-up copy of your 1.7.4 installation
INSTALL THE BACKUP SOFTWARE AND USE IT!