Mapped drives won't reconnect at logon in WinXP Pro Network

I've seen this problem described in this group, but in slightly different
specific settings. Up until a few weeks ago, I was able to have all of my
mapped network drives reconnect automatically at logon. For some reason,
this usually doesn't happen anymore; sometimes some of my mapped drives do
reconnect.

All of the computers of concern are running Win XP Pro w/ SP2, both before
and after this problem surfaced. We use a peer-to-peer network, not a
server-based network. By default, when I map a network drive, the selection
to reconnect at logon is selected. I do not connect using a different user
name and password. As far as I know, no other computer on the network has
developed this problem.

The only software changes that MAY have been made were the installation of
software to check the validity of legal citations; and an update to my
non-Outlook calendar/email/etc. program (I use Time Matters 7). I also ran a
program called Registry Mechanic, but I believe this was AFTER the problem
first surfaced.

Is there some setting that may be at fault?