use a different browser and/or a different (wired) connection and/or a download manager and/or a different link.
for the prodesigntools.com link:
if you follow all 7 steps you can directly download a trial here: Download Adobe CS5 Free Trials – All Direct Links without Akamai | ProDesignTools
and activate with your serial number.
if you have a problem starting the download, you didn't follow all 7 steps, or your browser does not accept cookies.
the most common problem is caused by failing to meticulously follow steps 1,2 and/or 3 (which adds a cookie to your system enabling you to download the correct version from adobe.com).
failure to obtain that cookie results in an error page being displayed after clicking a link on prodesigntools.com or initiates the download of an incorrect (eg, current) version.