Even in older versions, Krita provides a professional-grade experience for free:

A highly compatible editor that reportedly works all the way back to Windows 2000. Krita Artists Windows Xp - General Questions - Krita Artists

While official support began phasing out, some versions of 2.9.x might run if your XP system is fully updated with Service Pack 3 (SP3) and the Microsoft Installer 4.5.

This is widely regarded as the most stable and "last" version specifically tailored for Windows XP.

A powerful open-source editor that supported XP for many years. Paint.NET: An excellent, simple tool for quick edits.