Gigi can't install Photoshop because her "scratch disks" are full. Fortunately, for Gigi, Photoshop expert Burt Monroy is on hand to help out. It could be pointing to the wrong drive as a default disk. The chatroom found this thread from feedback.photoshop.com. The only solution there would be to install it on a smaller drive, but Leo says she could partition her current drive and install Photoshop to the smaller partition.
Gigi can use the Disk Manager utility to create the partition. She can right click on "My Computer," select "Properties", and she'll want to get the management console running. She should look for "Disk Management," and make a small 250GB partition, and leave the rest of the drive to the second partition. An easier solution would be to upgrade to the new Creative Cloud version of Photoshop, which handles the larger drives quite easily. She could get Adobe Photoshop and Lightroom CC both for $9.99 a month.