Windows XP was last released by Microsoft in 2001 and they stopped supporting it in 2009. They continued "extended" support until April 2014 - that means they kept fixing security holes. Now they don't fix security problems and it's less of a good idea to use it.
We are now seeing Microsoft's software development tools (which we use) abandoning support for XP and that is forcing us to soon stop supplying software that will run on XP.
SagePay who provide our payment handling services no longer trust Internet Explorer on XP as providing sufficiently secure internet communications. People wanting to buy Nuno or charts on XP should use Firefox or Chrome browsers instead.
If you are using XP you should seriously consider replacing it - not least because Windows is much better now than in 2001!
For now - Nuno does still run on XP including the latest release Nuno 22.214.171.124. The next release almost certainly won't.