Unable To Change Task Priority

I am using winxp pro. A few days , some of window services has changed the task priority to high level .It causing my winamp sound to be skipping, my firefox is stuck for 1 sec then load back normal during surfing. Playing game is hard since the game movement is jerky. Im pretty sure before this, all task in my winxp had normal priority. Please help me. I try change the window xp appearance to classic. But it still same. I also noticed during the jerky, the processor jump up to 100% usage. Also the mouse movement become slow during this jerky. Unable to atatch screesnhot . The processes which using high priority are:csrss.exe,winlogon.exe, ashserv.exe[Avast Antivirus Service], taskmgr.exe.