1. Make sure you exclude the BOINC directory and all subdirectories (or the BOINC Data directory and all subdirectories in BOINC 6 and 7) from being actively scanned by anti-virus and anti-spyware software. Only scan when you have exited BOINC.

4. Disable Drive Indexing.

3. Don't run Scandisk with BOINC on.

5. Update your motherboard chipset drivers, specifically those for your IDE or SATA controllers.

7. When you use use BOINC's CPU throttling function, you can run into the too many exit(0)s error. The advice here is to disable the BOINC throttling (set it to 100%) and reduce the amount of CPUs/cores for BOINC to use.
** Use at most 100.0 percent of CPU time.
* In BOINC 7.0, this is done through the option On multiprocessors, use at most xxx% of the processors.

2. Don't defrag your disk with BOINC on.

6. Disable the Time synchronization in Windows XP/Vista. Normally found under the clock (double click it in the system tray), third tab (Internet in English), uncheck the sync option.

If all is running as it should be, the BOINC client itself will place a vanishingly small demand on your GPU - certainly undetectable at your desktop. All the BOINC client does is to start and - crucially for your question - stop a science application supplied by one or other of the projects you are attached to.

If your general preference is "Don't use GPU when computer is in use", then BOINC should detect your usage, and send a message to the science application to

1) stop GPU processing
2) unload itself from GPU memory

There are occasional reports from the Windows world of science applications which fail to notice the 'stop' message, and continue processing when BOINC thinks they are inactive. From what you say about temperatures, it sounds as if your science app has stopped, but maybe it hasn't released the card's video memory? It might be worth discussing your problems with other users of that application on the project's message board.

OK, so what do you then mean by 'nothing'?
Else, what science applications do this explicitly?
You have taken into account, etc.?

Have you asked at the projects whose applications do this if this is something they know of, or that others see this as well?

I mean, in Boinc Manager\Statisics they both are flat lined at zero on host total. User total figures are correct on the i7-840. BUt in the i7-920, the only project which shows anything in User total is LHC@home. So, this is definitely whacky.

I have not gone to the projects because all of the projects are running not only on these two machines but two others, and they are both fine. Also, the Statistics show up correctly for both User total and Host total. So, I believe that the problem(s) are mine and not the projects'. Boohoo.

Clean driver installs didn't solve it.

How clean? Did you just uninstall the previous drivers and install the next, thinking that's clean? Or did you use something like Driver Sweeper, or Driver Fusion to thoroughly get rid of all old drivers and then install the new ones?

Just copying an OpenCL.dll in other places isn't going to work. It isn't just OpenCL.dll that makes OpenCL work. You'll need to install it with the drivers.

3. the HD5670 supports OpenCL, and would download and run OpenCL apps which are generally much more useful. But when the 4250 is detected using the method in (2), none of the cards are detected as OpenCL capable. The apps that were downloaded by the 5670 would show errors like this:
7/1/2013 22:47:15 | | App version needs OpenCL but GPU doesn't support it
7/1/2013 22:47:15 | Poem@Home | Application uses missing ATI GPU