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.
" My 755 on won't run XP." This is false. OptiPlex up to and including 620,745, 755,760,780,790 3010, 7010, 9010 will run XP. however XP is end of life end of support so it wont work on the internet. Youtube GMAIL etc will say your browser tastes bad and not let you in.
Models from 2000 to 2005 were WIN95, WIN98, 2000 MACHINES. The GX620 is from 2006.
The 620/745/755 has a label called WINDOWS XP vista capable. They started using VISTA in 2007 and WIN7 in 2009, WIN8 in 2012 and WIN10 in 2015.
The GX620 is ok the GX520 is a crippled GX620 with the PCI-E video slot and 2 ram slots removed.
GX620 TOWER will run MSDOS all the way thru WINDOWS 10 as will the GX745, 755, 760, 780, 790, 960, 980, 990, 3010, 7010, 9010.
3010 is the lowest model that is "SUPPORTED" via WINDOWS 10 but the older models work just fine.
The 620 in particular can take the hard drive from a 240 and install it and boot from it.
The chipset being different might cause a stop 7B message before reinstall.
That's why I recommended this specific model