Tested on Windows XP Professional SP2 and Windows Server 2003 R2.

First, click "Run..." on your start menu, type "regedit" and press enter.
The Registry Editor opens up, and you are presented with a long list of keys on the left.
Browse through the list to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\WPAEvents.
(If you can't find this key, you're screwed!)
On the right, you should see a stringz value named OOBETimer.
This is the activation stuff. If you change it, Windows will change it back within a few seconds.
Whatever its value is, change it to FF D5 71 D6 8B 6A 8D 6F D5 33 93 FD.
(At first I thought this was different for each machine/serial number, but it's always the same if windows is activated...LOL)
After you've changed it, right-click WPAEvents and choose "Permissions..."
A little window opens up with a list of user names. Click SYSTEM, and in the list below ("Permissions for SYSTEM") check every box under "Deny". Click OK to own windows. You don't even have to reboot lol.
If you're too slow windows might change it back, so just hit F5 to refresh and make sure it got saved.
If not, just try again. Microsoft owns for making it so easy to hack their software.
So, just keep on enjoying an activated version of windows. swapnil_009 over and out!

The problem most of you are probably having is when you get to the "Permissions" section, so i expand on this tricky area.

Under "Permissions..." for WPAEvents, Go into the Advanced settings.

UNCHECK "Inhereit from parent the permission entries that apply to child objects. Include these with entries explicitly defined here"

When prompted what course of action to take from here, click "COPY"

From there, Highlight the SYSTEM line, and click edit.

Deny all options, apply, and run a free copy of windows.