apronk,
I think you mean WriteValue()

I tested the following:
I am Domain Admin
I put my Domain Account (Domain\UserID) in the local group "Backup Operators" on my computer.
(I also tested this when I was NOT in the "Backup Operators" group)
In the registry, I created a new key "HKEY_CURRENT_USER\test"
Here is the script I tried:
;Script Options
If Not @LOGONMODE
    Break On
Else
    Break Off
EndIf
Dim $RC
$RC = SetOption("Explicit", "On")
$RC = SetOption("NoMacrosInStrings", "On")
$RC = SetOption("NoVarsInStrings", "On")
If @SCRIPTEXE = "KIX32.EXE"
    $RC = SetOption("WrapAtEOL", "On")
EndIf

;Code
$RC = SaveKey("HKEY_CURRENT_USER\test","c:\test.dat")
If @ERROR
    "SaveKey Error" ?
    "Error " + @ERROR + ": " + @SERROR ?
    $RC ?
EndIf

$RC = LoadKey("HKEY_CURRENT_USER\test","c:\test.dat")
If @ERROR
    "LoadKey Error" ?
    "Error " + @ERROR + ": " + @SERROR ?
    $RC ?
EndIf

?
@KIX ?
@SCRIPTEXE ?

Get $RC

Here is the output on screen
 Code:
LoadKey Error
Error 5: Access is denied.
5

4.53
KIX32.EXE

I presume an undocumented feature or a bug or so?