ok i finally wanted to try THE WITCHER finally but my downloaded files (i dunno which exactly as ithe installer says only Voice16 or Sound3 etc.) have some error, i redownloaded 3 more times all files, no luck!

i didnt have any prob with other games so far also big installers/files, only the witcher so far.

It seems you've had bad luck so far.
The installer is the first of the 7 parts (setup_the_witcher_enhanced_edition_2.0.0.12.exe).
You have to download all parts in the same folder and start the installer. If there's a checksum error, at least one downloaded file is corrupt. The only way to solve this, is to re-download the according file undamaged.

Maybe your Internet connection is unstable. Try to stabilize/unburden it, e.g. by using a wired LAN instead of wireless LAN and stop all other online activities (like chats, web browsing etc.).

SOLUTION

Redownloaded!