We need more details like what happens when you try to run ps7?
Does it give an error message?
Have you made any major changes to your computer like a new and/or bigger hard drive?
A quick thing you can do is reset the ps7 preferences file and see if that makes difference:
You can delete the preferences file manually by going to:
C:\Users\Your Name\AppData\Roaming\Adobe\Photoshop\7.0\Adobe Photoshop 7.0 Settings
and deleting the folder called Adobe Photoshop 7.0 Settings
(you need to have show hidden files and folders checked in your folder options to see the files)
Another thing to try is going to:
C:\Program Files (x86)\Adobe\Photoshop 7.0 (64 bit system)
C:\Program Files\Adobe\Photoshop 7.0 (32 bit system)
and right click on the Photoshop.exe and choose Run as Administrator and see if it starts then.
Also, have you installed the photoshop 7.01 update?