Where did you purchase the PSE8 from ? If its an ESD or a download ordered directly from Adobe you may want to refer to this link in order to download the same from Adobe. Also if purchased from Adobe directly when was purchase made ?
If you purchased the boxed product you will need the original disk.
If you purchased from the Adobe on-line store a download may still be possible.
Click the red Adobe logo at the top left of this page.
Then click My Adobe >> My Orders
Log in to your account using your Adobe ID
Expand your purchase history using the small black triangles to the left of the purchase date and if a download is still available for your product it will be shown to the right.