MS IE 1 was developed for Windows 95 Plus!, MS IE 2 for Windows NT 4.0, MS IE 3 was for Windows 95 OSR2 (essentially new OS, it even included it's own brand-new filesystem!), MS IE 4 was for Windows98 (but was rushed with Windows 95 OSR 2.5), MS IE 5.0 was for Windows 2000 (but was included as part of Windows 98 SE), MS IE 5.5 for was Windows ME (which wasn't initially planned - that's why it got 5.5 number, not 6), MS IE 6 was for Windows XP, MS IE 7 was for Windows Vista (and since Vista was horribly delayed so was MS IE 7) and MS IE 8 was for Window 7!
The first version of MS IE which wasn't developed as part of Windows development was actually MS IE 9.