I've been downloading the files from here:
devel package and upgrade packages are underneath the full version, which is the version I've been downloading. You must have something to upgrade before you can upgrade it.
XAMPP for Windows 1.5.5, November 26th 2006
XAMPP Windows 1.5.5
[Basic package] Apache HTTPD 2.2.3, MySQL 5.0.27, PHP 5.2.0 + 4.4.4 + PEAR + Switch, MiniPerl 5.8.7, Openssl 0.9.8d, phpMyAdmin 188.8.131.52, XAMPP Control Panel 2.3, Webalizer 2.01-10, Mercury Mail Transport System für Win32 und NetWare Systems v4.01a, FileZilla FTP Server 0.9.20, SQLite 2.8.15, ADODB 4.93, Zend Optimizer 3.0.2, XAMPP Security. For Windows 98, 2000, XP. See also README
Installer [MD5] 33 MB Installer
ZIP [MD5] 86 MB ZIP archive
EXE (7-zip) [MD5] 28 MB Selfextracting 7-ZIP archive
I downloaded the all three of them, checked the MD5 and they match. I also looked in "xampp\apache\bin" none of them contain a "php.exe"