won't go into standby

[Mike]

I have a brand new computer with XP home edition. I have
standby set for 20 minutes. It worked fine for awhile. I
did make some changes and then it wouldn't auto go into
standby. After about 3 days, I restored back to before I
made any changes and it was working again. SInce then, I
installed a firewall and disabled Windows Messenger. Now,
it won't go into standby again automatically. I have
played around with it and it will go into standby if set
to 2 minutes, but nothing else. I have turned off the
firewall and that doesn't fix it. I think something is
running in the background and prevents it from entering
standby. I did find a knoweldge base article that says XP
does maiantenance when idle and sometimes will not go into
standby at the requested time once every 3 days or so.
But it will not go into standby at all other than set to 2
minutes. Any clues on how to resolve or what could be
causing it?