laptop resuming from standby automatically...but why?

Folks, I don't know if this is a hardware or software question, but I'd like
to start here in hardware. Please correct (and forgive) me if I'm wrong. I
have a Dell Latitude D600 running XP Pro SP2 (latest updtaes), and more often
than not when I put the machine on standby, it automatically resumes some
time later--could be hours, usually it's when the laptop is in my bag between
appointments, where it heats up enough to melt any snacks in the bag! :-)

I've checked the wireless (and wired) network adapter properties, and
neither is set for "wake on LAN". Is there any other hardware, software, os,
or driver factor that could be influencing this very unwanted behavior? (I
don't have enough spare disk space to enable and use hibernate instead, while
leaving me a meaningful amount of drive space for other uses).

I'll add that when it came on last night, the battery was 90% full, so it's
not that it's awaking due to low batter warnings. I'm stumped. Any thoughts?
.