Free hard drive diagnostic software?

If the HDD is made by Seagate go to their website and download their Seatools package.If it is by WD, their package is Data Lifeguard. Other HDD makers may have similar diagnostics.

In each of those case you can get several versions. The have versions that are simply applications that you install on your main drive and run under Windows. But they also have versions that run with no functional OS installed, and I prefer these. For example, Seatools for DOS comes in two forms - a smaller one that can be fitted entirely on one floppy diskette you boot from, and a larger one that goes on a CD. The latter comes as an .iso image, and you need some CD burning software like Nero that can burn this disk image to a CD-R. Then you boot from that CD.

The "for DOS" versions boot from external media as above and load their own small OS into RAM and run from there. You do not need any functioning HDD in the system - just the drive for the diagnostic disk (floppy or CD).

Most of these suites have some simpler quick diagnostics and some tools to read out the HDD's SMART data. Then they have some more thorough tests that can examine ever sector of the HDD, but they take some time to work. In general, these tests are non-destructive for the data on the HDD.

Some also have tests and tools to make certain repairs that are destructive to data, and they will warn you before proceeding and seek your permission. If you plan to use them make sure you have another copy of the disk data somewhere!

ok, i have run some tests now. But is there a program that can run continuous tests, that i could run overnight, to ensure the drive is stable?

Many such diagnostic test suites have an option to keep running a series of tests in a loop until it is stopped. Look for that in your software's menu system.