If ever you need to find out if you can install Cognos to a specific platform, I'd suggest that you read the supported environments page as per belowhttps://www-304.ibm.com/support/docview.wss?uid=swg27014110
I've worked in IBM Cognos Support for a few years and this is where I suggest people look before jumping into this type of endeavour.
As you will find, Cognos 8.4 does not support Windows 7, and Vista only maginally
Microsoft Windows 7 Not Supported Not Supported
Microsoft Windows Vista SP1 Active Active for Browser and Client only
Microsoft Windows Vista Compatible Compatible for Browser and Client only
Tip to find the supported environments page ... google keywords ( IBM Cognos <version you are after, e.g. 8.4> supported environments)
If you want to install Cognos on Windows 7 I'd suggest you look at 8.4.1https://www-304.ibm.com/support/docview.wss?uid=swg27017522
Hope this helps for future reference.