well bro go to Edit -> Preferences -> Performance.
ok so since your comp is an old one it will say "No GPU Options Available in Photoshop Standard"
and to use scrubby zoom, you need OpenGL to be enabled and as your comp is old you will have to update your video card or buy a new one
it will look like this:
well so there are 3 methods to fix this:
1. Get a New Video Card
or
2. Maybe your comp is running slow? Is there too much space on the C drive? If so check
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otherwise
(hmmmm.... also you see the picture above, it says scratch disks and the space left in each, if you have connected photoshop with those drives, you should move photoshop to the drive with the most space, or just clear space using add/remove programs in the drive which photoshop is in especially if it has very little space :/
If your old comp is not slow, then try to change the memory usage options and increase the RAM usage to see what happens...)
3. Reinstall Photoshop, there is a 50% chance of it working when recently installed
otherwise you cannot use this tool as your comp is old and not updated
if the above do not work, then either just use normal zoom..
Or just use photoshop on your new computer...