When I log off, my computer freezes. I turn it off; I log back on. After my applications load, I get the following message:

The instruction at 0x73e68d8c referenced memory at 0x000300004 could not be read.

Nothing specific for that found

Any changes that either happened or that you perhaps made to your computer around the time that the problem started; particularly any software or hardware that you installed or setup.

Could indicate memory be going faulty

Try this app and see what type of results you get
http://oca.microsoft.com/en/windiag.asp

How long ago did this start? Less than 2 months? If so, consider a System Restore:

Start > Programs > Accessories > System Tools > System Restore

Follow the onscreen instructions to restore your system to a previous date. Choose a date back far enough to a time you remember it working normally.

I just got same error after updating a vista system with updates from June till now, obviously some sort of system error -

For me, the error sits there about 3 secs and disappears and continues to shut down

I personally am not going to do anything as everything does work

If I run across something I will post back to see if can figure this out

Did this start happening after updating?

As mentioned above, try a system restore point