http://blogs.technet.com/b/mmpc/archive/2010/09/01/rogue-msil-zeven-wants-a-piece-of-the-microsoft-security-essentials-pie.aspx

A new rogue has started making its appearance from compromised websites: Rogue:MSIL/Zeven. We received a sample (70be8ca73142922fd78acf2aafa9f141a977f15a) and a URL and began our investigation.

Let us say from the beginning that the guys behind this rogue like to copy big-time. They start by auto-detecting what browser the user is currently using, and then faking the malware warning page if the browser is Internet Explorer, Chrome, or Firefox. This is meant to be a social engineering scheme in order to trick the user into downloading and installing the rogue, relying on the user’s trust of his day-to-day browser.

http://krebsonsecurity.com/2010/07/experts-warn-of-new-windows-shortcut-flaw/
Experts Warn of New Windows Shortcut Flaw.
Researchers have discovered a sophisticated new strain of malicious software that piggybacks on USB storage devices and leverages what appears to be a previously unknown security vulnerability in the way Microsoft Windows processes shortcut files.
capable of infecting a fully-patched Windows 7 system,
http://anti-virus.by/en/tempo.shtml
Malware installs two drivers: mrxnet.sys and mrxcls.sys. They are used to inject code into systems processes
https://docs.google.com/viewer?url=http://www.secureblog.info/files/new_rootkit.pdf click Download (221K) or just Look.
http://blogs.pcmag.com/securitywatch/2010/07/certificate_used_to_code-sign.php
previously-unreported vulnerability in Windows, and that the malware had been code-signed using a certificate from Realtek Semiconductor Corp., a real company.

I use XP to. When you set up your operating system, did you create an admin? If not when you are installing make sure that you check the box to allow all users. This is usually in the setup program or installer, whichever the software company is using.

I came back to edit this post. If you are talking about today's giveaway, it should already be preset to allow all users. I did a little decompile on the setup earlier and have posted it here http://www.giveawayoftheday.com/forums/topic/1229?replies=11#post-7001 .... Take a look at the requestedExecutionLevel level under the manifest.

Check the box if available anyway if offered.

What program or game are you trying to activate?

Your title is misleading, title makes me think you cannot activate windows xp.

In the 2 PCs that have XP, there is no specific Administrator account visible so I run the setup only using the created user account with administrator privilege but every time, it installed only trial version and asking for registration key. I do not have any problem using Vista or Windows 2000. Is there a tweak in XP that you know to overcome this problem?