How can I identify a possible memory leak?

(I'm going to make a similar post on Microsoft's Vista Discussion Group, but figured it worth a try to post the message here, too.)

I'm finding that after using my XPS 410 PC (running Vista SP1) for a few days, I'm unable to launch certain larger programs; i.e., Excel 2007, Word 2007, etc.

When I attempt to launch Excel, I get an 'EXCEL.EXE is not a valid Win32 application' error.

When I attempt to launch Word, nothing happens. Task Manager reveals that WINWORD.EXE loads and then immediately unloads.

Task Manager indicates that there is plenty of physical memory available for use. No new events are recorded in any of the Event Viewer logs when I perform these actions.

When these symptoms begin to occur, I also notice that locking the desktop results in just a black screen instead of the normal locked desktop. The only thing on the screen is the cursor blinking where the 'enter password' textbox normally appears; still, I am able to unlock the desktop.

When these symptoms begin to occur, I also notice that performing an action that normally displays the UAC dialog box makes the screen simply go black; still, I'm able to press Alt-C to accept the prompt.

My only recourse is to reboot.

I've been noticing this for about a month or so. I suspect a 'memory leak' somewhere--perhaps with another application, perhaps with the video driver. Could this be a result of installing Vista SP1? Any suggestions on how to determine what the problem is?

PC: XPS 410
OS: Vista Ultimate SP1
Processor: Core Duo (2.4 GHz 800 MHz FSB)
RAM: 4 GB DDR2 RAM @ 667 MHz
Video: 256 MB Nvidia 7900 GS