Can you successfully boot into 'Safe Mode with Networking'? Reached by tapping F8 repeatedly while booting up and it's usually the second item on the menu that ensues. That mode should give you a working, albeit low-resolution, screen to work with plus the added bonus of an Internet connection.
If you can do that then try to initiate System Restore to go back to before all this happened. If successful don't forget to update everything. There are a number of ways of inducing System Restore, here are some:
1. Click on Start, Programs, System Tools, System Restore. (in some OS's Accessories/System Tools)
2. Click on Start, Settings, Control Panel, Help and Support, Undo changes to your computer with System Restore.
3. Click on Start, Run and then type restore or rstrui in the dialogue box and click on Run when you see System Restore as an option or click on restrui.exe if you see this file.
Whilst in that mode you could try downloading Stinger & Malwarebytes Free for running afterwards to make sure things are OK, the latter even updates and runs in that mode. Links and instructions here:
If all the above fails there's a post here with quite a few suggestions and a link to a removal guide.
Some of the software mentioned on that post can only be dealt with under the supervision of an expert on that forum.
Thank you very much for all your explanations. I have done everything and finally it works.
Perhaps, i must do something else again: 'catalyst control center : host application has stopped working' appears sometimes and some of my files are hidden.
Thank you again.
Run Hijackthis and post its log on one of the forums - all listed at the bottom of that first document I posted. They can analyse things and tell yuo what's wrong.