Figured it out eventually. If all else fails, read the instructions.....
The first time you install and activate your copy of PixInsight, it will create a special license file on your hard disk. The license file has the following name:
Linux:
.pixinsight-license
Mac OS X and Windows:
pixinsight-license
and is located on your personal folder. Normally, your personal folder is at the following locations:
Linux:
/home/<your_user_name>
Mac OS X:
/Users/<your_user_name>
Windows Vista and Windows 7:
C:\Users\<your_user_name>
Windows XP:
C:\Documents and Settings\<your_user_name>
where <your_user_name> is your actual user name on your operating system.
Put my licence file back in the XP location and I'm back in business