Did you already try the officially supported Windows XP mode? It's a free download from https://www.microsoft.com/download/details.aspx?id=8002 .
Be sure to have a few spare Windows 7 PC's as long as needed!
Would anyone know a site where I might find old computers for sale? A Dell Precision 390 with Intel Core2 Quad processor running Windows XP Pro sp3. They were sold 10-15 years ago. There's none on Craigslist within the 200 miles radius they allow.
Need it as a backup for a computer that runs some legacy software that won't install on even Windows 7.