An operating system wasn't found. Errorn

[NoahDG]

An operating system wasn't found error. Can't go any further

[TheOldMan]

That message usually means the hard drive has failed.  For a PC that old, not really that out of the question.

If Windows Vista was still installed, that might be a problem getting the media to reinstall.  You can try to make use of any builtin Diagnostics by tapping Esc before attempting a startup again.  Keep tapping Esc until a popup menu appears.  I have not worked on one that old for a long time, so if you do get a popup menu, post a picture of it.

Or

I would make a Linux live CD/DVD to test if that is really what is the problem.  Make the Live CD/DVD and boot the PC to that disk. It will not make any changes to the Windows environment but give you a chance to do some tests.  There are some tests you can run to see if the hard drive is working and it will also tell you if the hardware still is functioning.

I like to use Ubuntu  or Knoppix - http://knoppix.net/  Being that old hardware, you might need an older version of either one of them, but they will run on that old hardware. 

The Ultimate Boot CD is nothing but testing routines.  I used that in the past

https://www.ultimatebootcd.com/

It is old, so not sure if the download still works.