apt database locked

[D. R. Evans]

About an hour ago, I tried to upgrade to KDE 3.5.5. Big mistake.

94 of 95 packages installed OK. That leaves me with the adept icon in
my systray saying that there is 1 update available,

But when I click on the icon and enter my password, I get:

-----

Another process is using the packaging system database (probably some
other Adept application or apt-get or aptitude). Please close the
other application before using this one.

-----

I have logged out and restarted KDE.

I have even rebooted.

But still I keep getting this message. I imagine that it means that
somewhere a lock file has been created and somehow did not get
deleted, presumably because one package did not download properly --
but I have no idea which lock file it might be, so I can remove it
manually.

Any ideas anyone?