Let me try to clear up what the "security threat" means for you. Basically, the problem is that existing security vulnerabilities in the OS and related programs (like Internet Explorer) will not get fixed. Let's look at an example:
Example scenario
Let's assume that there is a (still unknown) bug in XP's picture viewer. This bug will make it possible for a specially crafted image file to inject malicious code into your system. So if you click on such an image file, you're computer might get infected with malware.
Now if such a vulnerability were found in, say, Windows 8, Microsoft would release a patch to fix it as soon as they learn about it. So if you use Windows 8 and keep it up to date, you will not be vulnerable to any malicious image files. But if you're still using XP, you will not receive the patch and will still be vulnerable to attacks targeting this specific vulnerability.
Ok, but this was hypothetical, right?
Well, not so much. Microsoft very recently patched a TIFF file vulnerability which allowed arbitrary code execution via malicious image files - for example by viewing an attachment in Outlook.
Oh my. But that's just Microsoft's software - how does this affect 3rd party programs?
Any 3rd party programs which have their own updating mechanism (like web browsers, PDF viewers, Java etc.) will not be affected, as long as you keep them up to date and they don't rely on vulnerable Windows functionality. For example, most browsers have their own image reading code. Still, you don't know for sure if a specific program you're using relies on potentially vulnerable Windows functions.
Bottom line
If you follow the guidelines given in the other answer, you should still be pretty safe for a while. But theoretically, a new vulnerability could surface any day, and would leave your system open to attacks. And even when using virus protection software and a limited user account, there remains a small possibility that a certain security hole makes it possible to bypass all your security measures.
4Who sent this email? Because it wasn't Microsoft. They don't even have your email. – Ramhound – 2014-03-04T12:20:57.843
@Ramhound Wel it seems like they do. it comes from windows[at]email.microsoft.com and links in it popint to microsoft website. – web-tiki – 2014-03-04T12:25:52.007
3As I said Microsoft is not aware of your email, even if they have it, they have no idea your still using Windows XP. My concern is you have clicked on a link not actually sent by Microsoft. You should never click on links contained in an email – Ramhound – 2014-03-04T12:27:48.783
1It is very"easy to fake the From: line in an email. As @Ramhound says, this might be some kind of online scam (getting you to pay for some "protection") or outright malware installation. Perhaps you should ask here (separate question!) for step by step instructions to thoroughly clean your machine and secure it. – vonbrand – 2014-03-04T13:07:11.833
@Ramhound and vonbrand I understand that this email might be fake, thx for pointing it out and I'll take care of it. this wasn't the point of my question. – web-tiki – 2014-03-04T14:17:04.100
@kinokijuf the question you are pointing to doesn't answer the "what should I do to protect my computer" part witch is main concern of my question. I had already seen it and found it intersesting but I still had no clear answer of what I should do. – web-tiki – 2014-03-04T14:19:21.753
@chadocat See now my answer again. – stderr – 2014-03-04T14:44:42.380
There will be an official pop-up message apparently, see http://www.dailytech.com/With+Windows+XP+Support+Ending+Soon+Microsoft+Uses+Popups+to+Encourage+Upgrades/article34436.htm
– maxpolk – 2014-03-04T17:51:31.597