If your running F@H as a service (I think its the same for the desktop version) you can open client.cfg in wordpad (Note it appears as one line with no spaces in notepad). It has all the settings in there Look for "priority=XX" and "cpuusage=XXX" under the "[core]" heading, change to "priority=0" and "cpuusage=100".
EDIT: From F@H forum
Your CPU has HT enabled so it's normal to see 2 CPU. And XP should show you that FAH is using 50% of CPU (it's just a XP bug, since FAH is really using 100%): if FAH is using less, then something is not good