System looping during restart: Windows Error Recovery!

My wife and I are senior citizens on fixed SSI income. Someone donated to us a Dell OptiPlex 960 Personal Computer. It has Windows Vista Ultimate Software. Unfortunately, they did not give use the administrators password. So we have been struggling for the past two months - we can not afford to have it repaired - to get on line and become part of the Windows 10 community. When we restart it, we keep getting a Windows Error Recovery looping. It apparently will not even access Windows Vista, but just keeps looping. We tried to go online at the local public library computer to see if we could do it ourselves. We are stuck. We have no reset disc or anything else. Will you please help us at least reset the administrators password and access the existing Windows Vista Ultimate software log on screen. Then maybe we can figure out a way to change over to Windows 10. Thank you.