Since Windows XP support ended in April 8th 2014, Microsoft won't offer any security updates or technical support for the product. It's insecure to use Windows XP since it hasn't received a update since 2014 (apart from the WannaCry patch in May 2017). Running Windows XP means that hackers can use security holes in Windows XP that haven't been patched since 2014.
Many web browsers have also discontinued Windows XP support:
Internet Explorer (6-8) on XP: EOL: 2014-04-08
Google Chrome on XP or Vista: April 2016
Mozilla Firefox ESR on XP or Vista: Supported until August 2018
Opera: Opera 36 is the last version of Opera for XP or Vista and will continue to receive security updates for the foreseeable furture
With an older, outdated web browsers you'll see less web standard compliance as time goes on and you'll again be exposed to more security vulnerabilities.
Internet security companies have mostly also dropped XP support so your choice of AV programs are limited as time goes on leaving you more vulnerable to malware and cyber attacks.
If your XP system is on a Intel Core 2 Duo or better with at least 2GB of RAM you should be able to install Windows 10 to replace XP. You can use programs like StartIsBack or Classic Shell (Classic Shell is no longer in development) to mimic a XP start menu to help with the transition to Windows 10. You can use SpyBot Anti-Beacon to block Windows 10 telemetry if you're concerned about Windows 10 spying on you.