Some display settings have an advanced button, in there on the monitor tab there is a check box "Hide modes that this monitor cannot display"  I do not know if this will stop the screensaver problem.

 

You might look for an updated graphics driver also.  http://support.dell.com/support/downloads/download.aspx?c=us&l=en&s=gen&releaseid=R126608&SystemID=LATITUDE%20D620&servicetag=&os=WW1&osl=en&deviceid=5260&devlib=0&typecnt=0&vercnt=2&catid=-1&impid=-1&formatcnt=1&libid=6&fileid=168332  latest from Dell

"Hiding" un-supported modes doesn't help (and in fact I was noticing it sometimes picks 59Hz which isnt in the list). Also there is a newer Vista only driver now available (Jan 07) which I am running. I also made sure I had the Dell monitor definintion file but that doesnt seem to help either.

Have you tried not using any screensaver? They are really not needed nowadays and use up battery power. Just set the screen to go off instead. Check the advanced settings on your Video card software, too.

Screensave is defined as required in the AD GPO (as a screen lock to be exact).

Why don't just press the Windows Key + L to lock the computer instead of using the wallpaper to do that???

 

And I think the problem is because a bad video driver, maybe if you try a lower resolution you will not have that problem... But I know looks better with that resolution...

Screensaver is required, so even if one forgets to invoke the desktop lock, it will happen anyway.

I would recommend to put to the computer to hibernate or sleep, but it has give many problems... so forget about that..