A GX520, GX620, 740, & 745 will run WinXP. You might find some with XP, or no OS, on eBay. However, the GX machines are 12 yrs. old and the 700 series at least 10. My 755 on won't run XP. Many of that series started with Windows Vista. A couple of YouTube videos skipped the part of how they got their 755 to run XP.
If using an SSD, Win7 has the ability for format and partition it when it's plugged in as a 2nd drive.