Infragistics is one of those companies whos runtime consists of a boat load of COM DLL's and they don't provide any sort of runtime redist be it an EXE/MSI or MSM ( Merge Module ).
Are you targetting only Windows XP and greater? If so, I personally would deploy all of the DLL's in your application directly privately and use COM-Free registratition through manifests that way you don't have to write to the registry and take a chance of conflicting with another application. Basically when your EXE is run windows looks at the manifest and virtualizes the COM information allowing your process to make the needed COM calls.
InstallShield 12+ has a wizard to assist in creating this.
http://kb.flexerasoftware.com/doc/Helpnet/installshield12helplib/IWPRegFreeComWiz.htm
If you still have to support Windows 2000 or you have requirements that makes the above not possible, I would go ahead and install the files to System32 as permenant components and extract the COM metadata into your MSI. The component wizard will help you set that up.