Cannot get into DFU mode

Hi all,
first of all sorry for my English. I try to tell u what's wrong :)

Yesterday i bought iPhone 3G. Everything was working but pervous owner dont delete his massages and contact. I click "delete all data" or something like this. My phone was down.
I didint know that it was jailbreaking :( I dont know what soft it have too.

I'm trying to fix it all last day...
At start i had only Apple logo and my computer doesnt see plugged phone.
Now i have a cable and iTunes logo and my comp recognize a phone. I was trying to fix it by iTunes update but at 60% i have 1015 or 1604 error...
I was trying to fix it by redsn0w but i cant put in in DFU mode because i cant turn it off when its plugged to computer.
I have windows 7.

People give me advices that i should use TinyUmbrella or sn0wbrezze but i cant put it in DFU mode.
I can make a video if it helps u to diagnose my problem...
Please, if somebody can send me link to some tutorial or clearly write what should i do i will be grateful. I'm so tired right now... I worked on Linux, I downgrade soft in Samsung phone and never had that big problem...
One more time sorry for my English.