I do not know which version of Hfss are you using, but all the details should be in the
license.dat file, located in your admin folder.
Generally speaking, HFSS is very memory hungry, part of the reason is that it is a finite element method program resulting in huge ( but sparse) matrices.
During simulations, monitor your swap file usage, you need to monitor if you are using swap space.,
Ideally, use Quad core CPU with 16 GB of Ram and WIndows 64 bit.