Have you compared your system against spec benchmarks for your CPU/Mainboard combination?
Try
PCMark04 then compare against the library of other peeps' scores
I do have concerns about your overall Internet connectivity. A USB modem will drain something in the region of 20% of your system performance

We would advise you to replace the USB modem with a Modem that plugs into your Ethernet ports, RJ-45. Then you and your g/f can pluf directly into the modem/router. So
your system isn't being drained by your connectivity choice and your g/f connection sharing.