you may need to change game version not sure if is 1.7 XI uses but Steam uses another version not match XI other under what OS using ?
are you on TS (teamspeak) for talk through, PM me if want to go that route.
if new system the hardware not match current system for old drive has old system hardware may be the issue.
if so copy your old profile on a flash stick C:\Program Files (x86)\Activision\Call of Duty - Modern Warfare\profiles and your nickname folder to keep your rank.
other know your new system drivers are current and graphics card/chip. (Note: motherboard site check if they have a updater app will be easyier) update those before reinstall game