Banshee...go into Windows control panel and click on the Power Options icon. Under the power schemes tab there are 4 option boxes for monitor, hard disks, system standby, system hibernate. Set all options to NEVER.
I was having an issue whereby my frame rates dropped from 60 to mid-20's after my computer/monitor were allowed to go into sleep/standby mode. Professor Skuzzy mentioned a bug with WinXP that affects frame rates when the monitor or computer is allowed to slip into standby mode. He was right. I reset all options to NEVER and my issue was solved.