A "Windows Update" modifies the existing operating system to close (or disable) a pathway within the structure of the operating system itself, that allows a malicious code to enter and do something it was not intended to do.
"Anti-virus" updates don't change those pathways (since the operating system is only known to Microsoft), but instead, those updates are essentially a list of known malware codes: a bit like a specific pattern. In theory, it would stop that code from "running".
This approach has the disadvantage of being one step behind the malware writers who constantly modify their code 'pattern' for the specific task of avoiding detection (among other changes). Even current OS "definition based" anti-malware defences have this 'after-the-fact' behavior.
There may be many "3rd party" security suites that will cover the XP system, but again: it will be after the exploit has been unleashed onto the 'Net, discovered and back-engineered, and therefore already infecting systems around the globe.
The above covers the realm of the operating system (XP), but additionally you must update all "3rd party" software as well. Those often provide these malware payloads with the power to sneak thru defences and gain access to those operating system flaws, and infect the device.
You can take your chances, but if you've been using XP for high-value on-line work, reconsider even connecting to the 'Net with it...even new OS's should be carefully reconsidered for that matter. There are so many malware payloads for Windows, they have stopped counting.
Why not get off the Microsoft crazy train all together?
Linux offers many flavors of free operating systems, and are far safer than any Microsoft one.
Not completely foolproof however: you must still keep your head in the game and watch for tricks used to get your information, or click links, and so on.
Here's one easy to use system: http://www.psychocats.net/ubuntu/installing but there are many others available.
"Dual boot" allows you to keep your XP 'as is', then just use Linux to do Internet tasks.
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ATM's and PoS devices would be out of your hands. Until the banks move to alternate OS's, patrons will be exposed to a wide range of exploits, which will almost certainly grow as long as they remain with XP.
Inquire at your bank and ask what operating system they are using (although they may be tight lipped about these matters.) Get (in writing) what their policy is regarding account theft from compromised ATM's. If what they regard as acceptable doesn't suit your standards, find a bank that does.