I am going to be brutally frank. This board is well beyond its expected lifetime and its components (including processor) are well beyond their Mean-Time-Between-Failures (MTBF) specifications. This hardware is not properly supported by Windows 10 and some of the slowness is the result of its drivers running in compatibility mode. My recommendation is that you recycle this system and purchase a more-modern system to replace it. Many of the latest generation Atom processors will likely outperform the processor that you have in this board.
Sorry I didn't have a better answer, but this is reality...
This message was posted on behalf of Intel Corporation
Correct, Scott's answer is accurate, the compatibility of the system with Windows 10* is probably causing the slowness in your Intel® Desktop Board DP35DP (Launch date: Q3'07).
You may also want to confirm that you are not experiencing any hardware failures (Memory, HDD, PSU).
Here you can find compatible OS' for your board: http://www.intel.com/content/www/us/en/support/boards-and-kits/desktop-boards/000005640.html
Here are the minimum requirements from Microsoft for Windows 10*: Windows 10 Specifications & System Requirements | Microsoft