Can't install Silverlight because it can't find installer

WARNING: Messing with registry entries can be dangerous. Anything you do with it is YOUR problem, not mine. Be sure to back up things before you delete things. I don't know if this will break things. All I know is that it allowed installation after it was removed.

1612 error for Silverlight 3.0

(you might want to remove the Silverlight 3.0 directory from Program Files first).

Go to regedit
[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Installer\Products\D7314F9862C648A4DB8BE2A5B47BE100]
(you might want to back up this registry entry before you continue)

delete this entry.
Now you can install 4.0 without error.

IPSEC prevents Windows Server 2003 to connect network

Problem: Network connection fails completely on Windows Small Business Server SBS 2003. Can't ping. Can't browse the Internet.
Reason: IPSEC service doesn't start. (Usually after SP2 install)

Problem: IPSEC service doesn't start because of some "only one" error on "port"
Solution:
HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Reserved Ports
add:
4500-4500
to the list
restart DNS.
start IPSEC.
You should have network connection.

Can't install Virtualbox Guest Additions

About ready to yell as I can't install 3.1.4 guest additions in VirtualBox XP guest. http://forums.techguy.org/windows-xp/235959-cannot-find-file-specified-during.html fixed it for me.
Problem: installation failed/file not found

Short short: RunOnce was removed in the registry as a key.