Adobe Photoshop Elements 10 forgets serial number

As per the title PSE 10 won't remember the valid serial number that I enter when I start the program. After a couple of days I have to re-enter the numbers or choose to continue a trial? I have a full version of PSE 10 on DVDs with the serial numbers for both Photoshop and Premiere on stickers on the back of the packaging.

What's going on here?