(I apologize my English, I'm italian)

I've a laptop HP zd8000, with W-Vista Premium, and it worked very well, till
I installed a new antivirus. I removed Avast Edition Home and I installed
Norman Virus Control.
Now, when I try to restart Vista, it doesn't work: if I click on "restart"
button (Riavvia sistema) it normally shuts down, but doesn't restart (it
seems freezed on black window).
Then I shut off with on/off switch (on computer), I wait for a while (3 or 4
minutes), then I switch on the pc and it starts (and it goes normally).

Can you help me?
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Thank You.
Friendly, Volosauro