To open Autodesk Inventor part (.ipt) or assembly files (.iam) in SolidWorks as features, you must have Inventor 11 installed. You can use Inventor View to import files as bodies without having Inventor installed
First question is what version of SolidWorks are you using (standard, professional or premium)?
If you are using Professional or Premium it includes "FeatureWorks" that will do the full conversion for you.
If you are using Standard version then you have the following options (and possibly more I don't know about):
To open an Inventor file (.iam or .ipt) in SW Standard version you either need a licensed copy of Inventor (of the version used to make the file or newer) on your computer.
As Kelvin gave a link to above "Autodesk Inventor View" (a free viewer program from Autodesk) will also allow this to work, but as kelvin said it will only create dumb solids (I have not tried this yet myself to see how well it works).
There are also other programs such as CrossManager (that Francis gave the link to below) that will do it - but you have to pay for a subscription for that.
If you have someone in your company that has Inventor and SW on their computer they could convert the program for you.
Another possibility, if you have someone else that has Inventor and you have the installation disk that you can install on your computer, they can export the license to you for Inventor so you can convert your files.
If you have the correct program on your computer, you can then just drag and drop from Windows explorer to SW UI and the file(s) will be converted.
If you drag in a part, that part will be converted.
If you drag in an assembly, every part in the assembly will be converted individually and the assembly will also be converted.
One note on converting, you can convert as a dumb solid or with all features.
I have not tried to convert an assembly and verified if it will allow each file to convert with full features - it may just make dumb solids.