For Yahoo! Messenger with Voice 8 users, it’s now possible to run multiple instances of Yahoo! Messengers and log into multiple Y!Msgr accounts with just a simple registry hack.

Multiple Yahoo! Messenger

If you prefer to edit and change the registry setting yourself manually, without using the above registry registration file, simply launch the registry editor (type regedit at Run command), and navigate to the following registry branch:

HKEY_CURRENT_USER\Software\yahoo\pager\Test

Then, on the right pane, right click on the registry editor and select “New”, then create a new DWORD registry value. Name the new registry key as Plural, and assign it the value of 1 (decimal). If some case, the value of 2 should also works. To disable or disallow multiple instance of Yahoo Messengers, simply change and set the “Plural” registry key’s value to 0, or delete the “Plural” key.

Wanna tell your friends about the do's and dont's in your computer when they login in your absence. Well you can do it pretty easily by displaying a legal notice at system start up.
REGEDIT
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\policies\system]
"legalnoticecaption"="enter your notice caption"
"legalnoticetext"="enter your legal notice text"


How to Enable Quick Reboot

This Hack will allows you to use quick reboot function to restart Windows immediately without shutting down. When its enabled, you can restart the Windows by press Shift + Ctrl + Alt + Delete.

Navigate to:
HLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon

Create a new or modify the existing String value called "EnableQuickReboot" and set it to "1" to enable it.

Note: USE this METHOD only when the system has stopped responding.