Control panel, then Display. Go to the screen saver and adjust the setting to a longer duration.
Forgotten if you have the "power options" as well under XP. If you do, go there, under power schemes adjust the time that it is idle then goes into hibernation in this area as well.
If the computer is taking this long to boot up, have you searched for spy bots and such? Cleaned up your hard drive and defrag'ed it recently? If not these can help the boot up time.