I have had an old symptom crop up again on my Vista Ultimate SP1.
(This makes the operating system basically unusable for me.)

Here's what's happening:

1. I boot up Vista - used memory shows in the task manager Performance
tab as around 1GB on my 3GB RAM machine. The usual start-up apps:
Sidebar, McAfee, Windows Update, etc. This seems fine to me, and the
machine works fine too --- but only for awhile..

2. Even if I don't run anything beyond the boot up programs and Task
Manager, and just leave the machine to itself on the table, after some
time the Used Memory will start to climb, 1MB at a time every 2-3
seconds until it reaches 2.8GB and the machine starts to falter
(missing fonts displayed, locked-up, etc.)

How can I track down which task, service, driver, etc. is using this
memory?

The used memory is not showing up in the task list, which continues to
total to a reasonable amount and does not increase.
There do not seem to be any strange or new tasks in the list. In fact,
I can kill tasks one by one until the machine is barely there, and the
Used Memory is still almost the entire 3GB or RAM. The only temporary
"cure" is to reboot.
This leads me to believe it is a memory leak in the kernel (Windows
subsystem) or a driver.
However, I haven't added or downloaded any new drivers lately. (This
is a Dell Precision notebook)

The last time this was happening (about a year ago), I was able to
disable the Windows Search service and the problem went away, since
this was triggering overnight and crashing with "Out of Memory"
somehow.
Trying that same tactic now is not working, however, so maybe that was
not the problem after all.

Is there a tool that will show me who's using all this memory? (2 GB
of it!)

(Remember, the extra 2GB of "Used Memory" doesn't show up on the task
list or the Resource Monitor under Memory (except as the "% used
memory" graph, which goes toward 100% and crashes the machine.))

Or... might this be an issue that SP2 will fix in Vista?