That's an old piece of iron. But I owned 2 at one time, now down to a single d5000t HP core2quad.

-> Start with a deep clean of the insides. Then pop off the CPU cooler to wipe off the old compound and apply fresh. While the cooler is off give the fins a good blast of canned air and swab clean.

Button it up but leave the case cover off.

Our last 2 core2quads got this treatment and run well under load. But at that age it's not a sure thing.
Both are running Windows 10 Pro because... they came with W7 Pro.