XAMPP is an integration facility for Apache, mySQL, MercuryMail, FileZilla, PHP and Perl.
It facilitates an easy and simple installation process on any Windows operating systems. The core of the system is the Apache Portable Runtime (APR), which enables the Apache core to run on more or less any system with a C compiler.
The only point that may be deemed an issue is that all of the modules run on standard server ports and as such would conflict with IIS if you tried to run them on the same ports.
Simple solution is to run the XAMPP modules on different ports to any that IIS uses. This is quit easily achieved and documented by using the various configuration files. Also an xampp-portcheck.exe file is included to check for port conflicts. I also found a site that has a different solution if you want to run both on the same port 80:
http://www.experts-exchange.com/Web/Web ... 18472.html
You won't break anything by trying out XAMPP as I am sure many who frequent these forums have done already.
I found a site that has some good comparison information about IIS6 and the latest Apache2 servers. IIS6 is only available in Windows Server 2003.
http://www.serverwatch.com/tutorials/ar ... hp/3074841
If you have specific issues after installing XAMPP on WS2003, please feel free to post them on these forums.