Please provide more info about your system.
Its a mac, i duno much more about it then that ... its about 3 years old
ive got photoshop on it fine and another user on this mac has used the trail before on a different account so its worked before
I would suggest run the cleaner tool : http://www.adobe.com/support/contact/cscleanertool.html?promoid=GYTAU
Make sure your user have read and write permission on the application and library folder of your hard drive.
Then try to redownload the software and install.
If it fails then run the disk permission from disk utility.
Another one you can try is to create a new admin account and downlaod and install the software on it.
Hope this helps.
Thank you shall try that