I just tested the power CMS Builder download and it downloaded successfully for me.
Do you have a virus scanning program that is scanning the downloaded files? Try turning that off.
make sure you delete the files you have already downloaded before you try downloading again.
Also, Empty the browsers cache
after the file has finished downloading, don't double click the MXP file to start the install, instead, open the Extension Manager, click the install button and browse to the MXP file you downloaded.
make sure you are browsing the correct download folder for your browser.