UPDATE
Today I received an email from Xara and Magix stating that it has now been activated on both pc's! However, like the last time there was no apology for the inconvenience. In response to the question by "Acorn" both pc's had been upgraded to Win 10 since the initial install. The computer which locked me out was upgraded to 10 about a month ago which was an automatic upgrade with the transfer of all files and programs completed through that progress. All the installed programs, except for an old one which was incompatible with 10, have been running correctly, as indeed Xara was until yesterday. Even if 10 had been installed as a clean install I should have been allowed to install Xara as non of the hardware had changed. This problem seems isolated to Xara 11 as v 7 and v9 have continued to run when the OS was upgraded from XP to Vista and again to 7. Thanks to all who responded, but I have zero confidence in Xara/Magix and will not be buying any more of their products in future. That only leaves Corel as the only viable alternative as Adobe now require an annual subscription. That's why I am still using CS5. Pity, as Xara DP is a good program but a bad company.