You cannot get support from Microsoft, and official updates are usually blocked, leaving the system vulnerable to new security threats. Verdict
Third-party ISOs can contain pre-installed malware, spyware, or backdoors [1].
Instead of using a modified, potentially insecure OS, consider installing a (such as Lubuntu or Linux Lite). These offer: Excellent performance on old hardware. Far superior security. Full, official functionality. If you can share:
Extremely low RAM consumption (often under 500MB-1GB at idle) and reduced CPU load [1].
I can offer better, safer alternatives than a modified version of Windows.