Excuse me PC ignorance, but I don't know any of this jargon, what does that mean?
Open up Steam Settings and disable the In-Game Community and then right click your Status and switch it from Online/Away/Busy to Offline.
To set it to low priority open up task manager, click processes, find Steam.exe, right click, Priority -> Low.
Can you tell us your GPU (Graphics Card) and Operating System(Windows XP/Windows Vista/Windows 7/64bit or 32bit)?