How unsafe is Windows XP

[#1]

I suspect that if you're at home behind a properly-configured firewall, there is very little additional risk from continuing to run Windows XP after April 2014. The vulnerabilities which Microsoft would otherwise be fixing with security updates are typically in services listening on open ports and by being behind a firewall, they won't be open anyway.
The risks I'm more concerned about these days are USB stick/memory card or drive-by infections from things such as Java plugins running inside the web browser, or vulnerabilities in the web browser itself. Mozilla, Google and Opera bound to ditch Windows XP for Firefox, Chrome, Opera Browser at some stage, but until then just enjoy using your PC. Just run an up to date virus scanner (again, until they stop supporting it on XP) to protect yourself as much as possible.
Having said that, I wouldn't want to run XP in a business environment. I know many businesses who are, and I'd bet good money on few of them planning on paying Microsoft for extended support.

[#2]

It will be a real pi**er for regulated industries like finance
Things like ATM's running XP will have to be updated/scrapped.
In these sorts of industries compliance will be mandatory, and having an OS no longer supported by the manufacturer in live use on your systems will be an instant fail.
As regards latest viruses - try Cryptolocker
This is a humdinger
once loaded on to your machine via an email and a dogdy attachment which someone clicked on.
It works silently in the background encrypting all your files using a public key.
Then it demands money with menaces via bitcoin (untraceable) and gives you a pay up by date.
It appears from the web that if you pay up you do indeed get your files back as it uses the private key attached to the public key derived form your machine to do the decryption.
But if you clean the machine you are left with the encrypted files and no way of getting them back
You have also by cleaning the machine removed the virus so you have removed the "pairing up" and even if you now want to pay up you can no longer do so.
If your time to pay up runs out I suspect that either you will be unable even if you want to pay to do so or the amount to pay has gone up by a few £K.
It also attacks any external/NAS drives which you have on the system via a drive letter.----NB that could mean backup drives and those files.
It appears that to date once infected and if you are unwilling to pay the only solution is to re-load everything from remote backups which are not permanently connected to the PC/network so were safe from being encrypted.  Some cloud backups which include version control so earlier copies survive "may" also be safe so it seems from what I read.
Victims should probably should zero the drive as well and do a clean install
The external servers used as the attack central are always changing - a bit like some complex constantly changing frequency hopping spy communication stuff so you cannot simply find a server and block it.

[#3]

It can only encrypt files to which you have write access.
If you read your email with administrator privileges, you must expect to have widespread damage.

[#4]

Since it changes files - primarily Microsoft Office and Open Office files which it can see  your comment about email with admin privileges is a total non sequiter
The virus gets in by masquerading as a pdf and if you are naive enough to click on it and your anti virus hasn't blocked it then it does it's dirty work

[#5]

There are technologies in the newer versions of windows such as ASLR, DEP UAC and some sandboxing that make them harder to infect than windows XP was.
The problem is a the technologies to try and mitigate infection get more advanced then so does the malware itself, Also nothing can protect against the user being an idiot and installing something nasty thinking it's cool similes or free music.etc
One point to make is it looks like microsoft may stop releasing AV defs for MSE on windows XP once it goes end of support so if you do plan on continuing use of windows XP on an internet connected system and are using MSE you might want to find another antivirus package.

[#6]

So XP is seriously infected with acronyms.