|if it's coming out of standby without you touching the mouse or keyboard, some operation in the background (eg a scheduled task) is probably waking it up.|
It could be a sheduled automatic virus scan, automatic defragmentation etc.
Take a look in the list of Scheduled Tasks that have been set up:
Start >> All Programs >> Accessories >>System Tools >> Scheduled Tasks
It can also happen if there's a hardware device which isn't compatible with the standby or sleep mode. The Event Manager may offer some clues as to what's causing it:
Control Panel > Admin Tools > Event Manager
Look for errors in the "System" category.