Reference:
http://en.wikipedia.org/wiki/XAMPPXAMP is a server with SQL and PHP and some other stuff.
Let's assume the OP is talking about the Windows version.
XAMP is an item for web developers and/or network administrators. The average user would have little need for this heavy duty developer tool.
IMHO, most users would be better off using a dual-boot Windows and Linux. The Linux can be a server and there would be little need for XAMP. That would preserve the Windows system. One would boot into Linux only when a server is needed for a project.
However, I have not used XAMP, so I do not know what, if any, harm nit could do to Windows.