Actually it was tied to Windows upgrades. It available for older versions of Windows but it was developed as part of development of new version of Windows!
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.
I don't know if it was developed as part of the development of a new version of Windows or, far more likely, it was simply released with each new version of Windows for maximum impact.
IE2 was the first version available for Windows 3.1, Mac OS, etc. Sure it was released at the same time as NT 4.0 but I don't think it was developed with it.
The big exception is IE4.0 which was deeply integrated with the Windows 98 shell (hello Active Desktop).
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.