In short, it sounds as if time is quickly running out. Quite frankly, if it does take the NHS until April 2021 to migrate all of its PCs to a modern operating system it's far from ideal, as Microsoft will have dropped support for Windows 7 long before in mid-January 2020.

With the ransomware outbreak dominating British headlines and living long in the public's memories, the UK government was keen to be seen as taking action to support the NHS and protect patient data, and in April 2018 a £150 million plan was announced to upgrade all NHS systems to Windows 10 'over the next three years.' Windows XP may not have contributed much to the NHS's WannaCry woe, but it did underline the sorry state of the organisation's IT infrastructure which had historically not received as much funding as it deserved. The NHS can ill-afford to suffer another attack like WannaCry, which an investigation found affected 34% of NHS trusts in England, causing 19,000 appointments to be cancelled, and significant costs related to IT support and data recovery. Reboot after computer has been renamed, and try joining it to the Domain again.The fact that 2,300 NHS computers are still running Windows XP is, obviously, not great news. Next download and run SysInternal's NewSID Application Make sure the machine is joined to a 'Workgroup' instead of 'Domain' (If you've tried to make it be in the domain and it failed but still shows as being in the domain, disjoin and reboot).

Sounds like you need to generate a new Security Identifier (SID).

Windows xp computer