1. copy your important files from the computer. They might be lost, while you format the computer.
2. You must have product recovery CD with computer.
if you do not have it, then you can make it from your computer. Use data recovery (Wiederherstellung) option to make the necessary cd/dvd. Its not hard, once you find the recovery management program, it says just says to enter the cd/dvd, the necessary files will be copied. Take care that it may need more than 1 dvd.
3. Once you have recovery cds/DVDs, insert first cd/dvd in the computer and start it new.
4. In the beginning, when computer starts you will see in the screen "Press F2 .... or F8 in some laptops", press that bottom.
5. You will see a the screen background black. It asks for source. Go to cd/DVD with the arrows up and down.
6. Follow the instruction it may take about 1 hour.
Its not hard, if you have recovery source.
I hope it is helpful to you