Only certain versions of iSeries client access are supported for Windows 7, namely V6R1:
http://www-03.ibm.com/systems/i/software/access/windows/supportedos.html
and only Enterprise/Ultimate SKUs in the Windows 7 family. If you installed an incompatible version, you might need to use System Restore to back out the install. And the shell extensions don't work on 64-bit and the /separate trick was removed in Windows 7 I believe, so you won't be able to configure a 5250 session from the IBM Navigator (well, at least not easily).