What comes next
Once it's working, the rest's just fine-tuning. The VM is already on SP3, but still, there's the usual tired old round of updates to do: it'll probably take a few passes through Windows Update. You can save a little time by going into Control Panel | Add or Remove Programs | Add/Remove Windows Components and removing absolutely everything except IE.
You don't need Media Player inside your VM and you don't need to waste time updating it, either. Assuming you've got lots of RAM, I recommend giving the VM 3GB – it's about all XP can use – and disabling the pagefile.
Once the VirtualBox extensions are installed, XP runs just fine... (click to enlarge)
Unless you have legacy intranet apps that require IE6, you can save some more time by downloading and installing IE8 before XP has a chance to download and install umpteen hundred patches for the old version.
Windows Update will take hours anyway, but since it's a VM, you can use your host OS for other things while the update chugs away. If you don't have a preferred antivirus, MS Security Essentials will do the job. Don't forget to enable Automatic Updates, after which, if you don't need IE, you can remove even that... and you're done.
The VM is just for your indispensable Windows apps – the idea is to use the host OS for Internet access, email and everything else. Don't access the internet from the VM. Now Microsoft has stopped updating XP, it won't be safe - the XP Mode site carries a hefty warning from Microsoft saying how it, like Windows XP, no longer benefits from technical support.
The last step is to enable VirtualBox's desktop-integration features (VM menu | View | Seamless). This is where it helps to be running Linux distro Ubuntu as I did, with its vertical Launcher. This lets you leave XP's taskbar at the bottom of the screen and they won't get in each other's way. If you prefer the more Windows-like environment of Mint, say, then you can make XP's taskbar vertical, or put it at the top, so that you can easily get at both.
Windows XP and Ubuntu in perfect harmony, thanks to Microsoft (click to enlarge)
OK, this is not a perfect solution, but it is quite a low-hassle approach plus the tools don't cost anything - handy in these cash-strapped times. But you do reclaim the support safety net.
What's more, this approach doesn't need antivirus or antispyware, and comes with an integral firewall and drivers for pretty much all current hardware. Meantime, you also get a fresh, sandboxed, clean and virus-free copy of XP Pro. It runs Windows apps with near-perfect compatibility, right on the host's desktop, complete with a shared clipboard and so on.
The best bit? You can continue to run Office for Windows or other line-of-business software, while transitioning gradually to some other solution – native Linux apps, Windows ones that will run cleanly under WINE (meaning that you can dispense with a Windows instance altogether), or possibly even VDI. Ubuntu comes with a perfectly usable Remote Desktop client, Remmina, so you don't even need a copy of Windows for that.
That's one way of escaping the XPocalypse. ®