QUOTE (TranceEnergy @ Jun 8 2008, 11:48 AM)

Alas MS fails to describe it properly...
Actually, Microsoft did explain it to you in plain English and quite clearly I might add.
QUOTE (MSKB 310516)
Syntax of .Reg Files
A .reg file has the following syntax:
RegistryEditorVersion
Blank line
[RegistryPath1]
"DataItemName1"="DataType1:DataValue1"
DataItemName2"="DataType2:DataValue2"
Blank line
[RegistryPath2]
"DataItemName3"="DataType3:DataValue3"
They also very nicely explain exactly how to delete registry values:
QUOTE (MSKB 310516)
Deleting Registry Keys and Values
To delete a registry key with a .reg file, put a hyphen (-) in front of the RegistryPath in the .reg file. For example, to delete the Test subkey from the following registry key:
HKEY_LOCAL_MACHINE\Software
put a hyphen in front of the following registry key in the .reg file:
HKEY_LOCAL_MACHINE\Software\Test
The following example has a .reg file that can perform this task.
[-HKEY_LOCAL_MACHINE\Software\Test]
To delete a registry value with a .reg file, put a hyphen (-) after the equals sign following the DataItemName in the .reg file. For example, to delete the TestValue registry value from the following registry key:
HKEY_LOCAL_MACHINE\Software\Test
put a hyphen after the "TestValue"= in the .reg file. The following example has a .reg file that can perform this task.
HKEY_LOCAL_MACHINE\Software\Test
"TestValue"=-
To create the .reg file, use Regedit.exe to export the registry key that you want to delete, and then use Notepad to edit the .reg file and insert the hyphen.
Both of those quotes come directly from the MSKB that
you linked.

Now for what you want to do, it's be exactly as GrofLuigi has said. Let's say you have a test application that you want to stop from starting up with the computer for every user account. You would put this into your .reg file:
Windows Registry Editor Version 5.00 = Windows 2000 and up
REGEDIT4 = Windows Me and below.
So since you're working with Windows XP Pro x64, you would have:
CODE
Windows Registry Editor Version 5.00
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
"MyStartupApp"=-
I would advise that you slow down and read through things thoroughly before jumping into things. When you edit the registry without knowing what you are doing, there is a great potential to cause problems.
If anyone is unfamiliar with the registry, it's best to leave it alone.