Chnaged computer, trouble

I have used Cubase Le4 with Presonus USB Audiobox without problems for a long time. Now I have changed to a new (faster) computer and have problems. The processor “peaks” when using low latency which never was a problem in the past. I have a feeling the problem is related to Windows 7 64bit but am not sure. Any proposalt? I have downloaded the latest version of the driver for the Presonus unit.