It is prehistoric level of ancient. The release happened only because they fixed an IE component that existed also in the newer versions. XP support is over. It has been a security hazard for years of course even before that. Even if you attempt to surf 'safe' sites, hackers can infiltrate an ad host so that one of the adverts you see on a completely harmless website has malicious code injected in it. Nobody else will now, except you whose OS has not been patched against the attack. Then once the exploit gets in your local area network it can start to bombard also other devices in it, possibly drop an other kind of exploit that can't be remotely executed but works inside the lan.
I would not surf at all on that computer (but then again I don't surf with any windows machine, period).
Well, in the case you don't have a choice but run XP due to hardware/software combo and you'd like to use the same machine for occasional web surfing...
You could:
Use browser other than IE (note that most will drop support for XP in the future)
Use decent anti-virus and firewall with HIPS (MSE won't cut it)
Continue updating your Win XP -> Windows XP POS systems will have security updates available until 2019.
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What is the difference between POSReady 2009 and Windows XP Professional?
Windows Embedded POSReady 2009 contains the following Embedded Enabling Features:
File-Based Write Filter (FBWF), which redirects writes to disk to RAM and helps protect the underlying OS image.
USB Boot, which allows installation from a USB key.
Office productivity applications are not licensed to be run on POSReady.
POS for .NET is included with POSReady.
POSReady does not contain Help files, which reduces its footprint.
Both products use the same management software.
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Simple registry hack allows POSReady 2009 updates to be installed on regular Win XP SP3 (until MS plugs workaround).
http://www.sebijk.com/community/board15-other/board73-tutorials/2985-getting-xp-updates/