Normally in XP or Server2003, I'd either go into control panel and adjust the display settings, or I'd run DESK.CPL (or desktop.cpl in 2000 or 98 or whatever).
However all desk.cpl does in 2008 and Vista is give you screen resolutions. The only CPL files I found were INTL and TIMEDATE. Since you don't have control panel or "Personalize" in the command-prompt-only Server Core, how do you adjust those settings?
I'm training on it right now and am running 2008 on a VM/Virtual PC, but it keeps going to screensaver and asking me to log in each time. If it was a production server or connected to the internet I would understand the security risk but neither of those conditions are being met.