Sounds to me like a case of not knowing how certificates work.
Very simply stated: Each certificate has a start- and end-date. After the end-date it automatically becomes invalid. Also, a Certificate Authority (CA) is used to verify the validity of a certificate. CA certificates have a start- and end-date too.
Operating systems always come with a set of those CA certificates and usually do not receive updates regarding those CA's.
Your old XP system will not have received many updates and if you didn't add updated CA's yourself, you will see many more browser messages about websites not being able to verify that they are safe to use. And if you need to resort to adding/removing CA's yourself, how sure are you that those updated CA certificates have not been compromised in any way? That's a rabbit-hole you hardly can reason yourself out of.
And you should also be worried that if you go to banking websites that still show up as safe in your browser. It means that these accept out-of date CA certificates as valid. That might make you "feel" safe, but in reality the banking info going through your system is open for 'man-in-the-middle-attacks' and all the consequences that come with that.
CA certificates are usually valid between 5 and 10 years. Certificates bought by users/companies are almost always 1 year valid. Free certificates are commonly valid for 90 days or less. There are good reasons to have validity period limitations. The main one is that shorter periods mean (much) more safety for the end-user.
Windows XP is falling dangerously behind the times. Not only because Microsoft doesn't make fixes for it anymore, not only because it's code-base is very well known in the wrong circles, but also because of a lack of updates on CA certificates.
Previous posters already said that every time you start up your XP computer and do anything on the internet, you are putting yourself at serious risk.
No matter how much you like XP, you should only run that operating system in a container (like a virtual machine) that is not allowed to go onto the internet. Even if a new system does not fit in your budget, An old Windows 10 system isn't too expensive for most.
Even though Windows 7 is also out of Microsoft support, it is still safer to use that than Windows XP. Windows 7 computers are practically given away nowadays. Donationcoder.com and its certificate aren''t the problem, your geriatric system setup is. Not by your fault, let me be clear. XP has out-aged the internet for quite a while now.