4 GB Ram, can be upgraded to 8. ...
I don't know what OS and what version of PS you are using, but as soon as my computer wakes up, it immediately uses 4.35 G out of the 64 Gig of RAM I have installed. If I fire up PS (with no image loaded), AI, a browser and a few other utilities, my RAM usage goes up by at least another 2 Gig. The minimum RAM requirements for the newer versions of PS is 2 Gig (and they recommend 8 Gig), so if your setup is anything like mine, having only 4 Gig of RAM, you're probably right at the hairy edge of not having enough RAM, then when PS is in the middle of loading, it runs out. Also, you didn't answer my question about whether you tested the RAM that you do have. Also, by any chance might you still be running a 32 bit OS? If so, upgrade.
The computer is a Dell Optiplex 780. Video is Intel Q45/Q43 Chipset. Where can I find the scratch files? ...
re: scratch files:
http://bfy.tw/6xNN .
Also, YOU (not me) need to go through the trivial amount of effort it takes to check for compatibility between your video system and the version of PS that you are running. At the very minimum, have you
updated your video drivers?
I am in the process of buying a new HD just for scratch files. ...
That's a nice idea, but if it were me, I would first check everything that I mentioned, NEXT spend money to get more RAM, and only then purchase another HD.
The voltage and everything should be fine, it came with the computer. ...
That's not true if something has gone wrong in your system since you bought it. Just go measure the voltage. It will take you a few minutes to do and cost you nothing:
http://bfy.tw/6xNT Doing so may eliminate one more of the many possible sources of problems.
AVG Internet Security scans my computer every startup and normally doesn't find anything in the way of viruses, a tracking cookie here and there, but in the life of the computer, maybe one detection and a few false positives. ...
Good!
No, I have not reinstalled PS. and it is the Photoshop from Adobe's website, just have it blocked with my Firewall to disable license check.
Given your low RAM situation and the many other possible causes for the problem, including the simple fact that your computer is a bare-bones entry level system, probably never intended to work with serious programs like PS, I would strongly suggest that you first work on each of the things that I mentioned above (including getting more RAM, updating video drivers, etc. etc.), and only then, reinstall PS.
Good luck.
Tom M
PS - Just thought of another couple of maintenance issues. If you haven't cleaned your registry lately, download the free version of CCleaner and do so, or, even better, just start from scratch by formatting your HD, and reinstalling your OS and PS. If you haven't run chkdsk c: /F lately, do so to make sure your HD isn't developing problems.
PPS - Haven't we already been through this a couple of months ago?