MATLAB 6.1 (R12.1) and 6.0 were released before Windows XP was finalized and thus was not validated under Windows XP. Thus running R12 on Windows XP is not supported.
For the list of system requirements for R12, see the system requirements page here:
While not supported, it is possible to run R12 on Windows XP. See the workaround below for more details:
The version of the dongle device drivers MATLAB installs are not supported on Windows XP and will not work on XP. To fix this, you can install a newer version of the drivers using dongleutil.zip, which is attached to the solution below. This utility allows you to install, remove, and just check what version of the dongle drivers you have installed.
To install the newer version of the drivers, simply download and extract the .zip file attached. Once extracted, run dongleutil.exe and select option 2.
After doing this, MATLAB should start. If you run into problems starting MATLAB after updating the drivers, see the related solutions listed below.