No, you don’t need to run out and getting a new tower before they’re no longer made.
Let me address your questions here more or less in order.
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Will Windows XP keep working after support ends?
This will be very much like Windows 98. If you’ve got Windows 98, it still runs. It may not support all of the software that you care about and you’re certainly not getting bug fixes, but fundamentally, it never stopped working.
The same will be true for Windows XP. When the end of support date arrives, Windows XP will just keep happily XPing along. You just will no longer get any important or even unimportant security updates.
No updates… is that an issue?
Well, yes and no. There’s really no harm in continuing to run Windows XP with the exception of what we refer to as “unpatched vulnerabilities.”
What that means is somebody discovers a bug in the software that could be somehow exploited by malware writers to insert or cause malware to be installed on your machine. If you’re running Windows XP past the security date, that unpatched vulnerability will remain forever unpatched. In other words, you will always be vulnerable to any malware that might want to take advantage of that.
How can I mitigate that problem?
If you’ve got anti-malware software that continues to work on Windows XP, that’s one way to at least minimize the threat. Good anti-malware software will probably catch the malware that tries to exploit those kinds of vulnerabilities.
The catch is how long will good anti-malware tools continue to support XP? This I don’t know.
The other factor is that as XP declines in market share, the actual authors of malware are now focusing their efforts and their attentions on things like Windows 7 or Windows 8. Those types of malware may not even apply to Windows XP anymore. In other words, you may actually be kind of more secure by having the older operating system that the malware authors aren’t paying attention to either.
So, ultimately, given what you’ve described – the fact that you’re really not doing that much with your machine and that you’re using an Ubuntu DVD to do your online banking – I think you’re in a pretty good situation and I’m not sure that there’s much I would change.