Lotus Notes 6.5.1 is not supported on Windows 7. In Windows Vista you could get Notes 6.5 to work if installed it in C:\Notes and gave the user full permissions to the folder.. Everything would work, except for Instant Messaging. This technique no longer works in Windows 7. The first supported client for Windows 7 is Lotus Notes 8.5.1 Fixpack 1. If even you get success to install any how, it creates problem later. You can also check with Microsoft Compatibility Center for confidence. One way you may try, to install it under Compatibility Mode. For this purpose, you have to require to install the compatibility tools for Windows XP or Vista under Windows 7.
Hope this helps you!