I have 30 rar files (part01 to part30) and i select them all and do
Extract Here. After that I'd had an ISO file but I tried to open it with WinRar it tells me that "file is in wrong format or it is corrupted".
What can I do to watch the video?
You should simply be able to select "part01" and select extract here and it should automatically find all the relevant parts and extract the file as needed.
As to opening the ISO file I would try opening it with another archive program like 7-Zip, or by writing it to a DVD (I am assuming it is a DVD video) using a program like CD Burner XP as an ISO file is typically a direct "image copy" of a DVD or CD.
If neither of these options work then it would appear that you have in fact downloaded a large amount of garbage disguised as a video file.
It appears that when WinRar was installed on your machine, ISO was not checked as a format for it to handle. To correct this, you may need to reinstall WinRAR after uninstalling it. Download WinRar from http://www.rarlab.com/download.htm and double-click on the setup file to proceed with installation - and make sure you check ISO to associate with WinRAR (when you see the select all buttom) during WinRAR setup.
After the reinstall is done, double click on the ISO file to display it's contents.