Ctrl+Alt+Del -> Failure - security options

Two or three times a day, while running no more than (say) Word and Outlook (Office 2007), my system goes extremely slow and erratic and I get the spinning circle that Vista has as its hourglass and all applications showing up with "(Not Responding)" in their title bars. While it is in this state, I naturally want to see what process is eating up the CPU (though the CPU gadget shows nothing amazing). However, when I press Ctrl+Alt+Del, all I get after a delay is a black screen and a dialogue box saying:

    Login process has failed to create the security options dialog.

    Failure - security options.

                            [OK]

Clicking OK leaves me with a black screen for a long time and I get returned to the frozen applications. I then have to reboot by holding down the reset for a few seconds (because gets its knickers in a twist resetting the soft way).

I have had this problem with Windows Vista Business and then installed Windows Vista Ultimate and have continued to have exactly the same problem.

My system is an Intel Core 2 Duo, 2GB, 2.66GHz, DELL XPS bought in January with XP. The soundcard (Soundblaster X-Fi) is on a beta driver, because the driver will only be available at the end of March and my two previous questions at http:Q_22407028.html and http:Q_22411870.html should be read in this context.

Should I be looking at SpinRite (suggested in the second previous question), which I didn't persevere with after my corruption was sorted out by chkdsk... or is my problem more likely to be something else?