Registry Setvalue doesn't always work!

[FenixEden]

the registry path is :
hkeylocalmachine --> software -> microsoft --> windows nt -->winlogon --> shell
I'm on windows 7

I run under admin acount.

I have no execption, neither first time exceptions...

As it is induced in the title...
Someimes it works, and sometimes not.
We always know if it did work because when it does, computer lags for a second or so.

Any clue?
If you don't have any, please at least tell me so...

I can change the value manually...
But I don't want my customers (cause i wanna sell that) to seek into the registry editor...

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EDIT
I have first chances exceptions, but they are at execution, not when I click my change shell button.