Thanks to all.
@Micropage. Yeah, but in a netbook w/ 1 GB RAM an resident AV may be too overkill here so I run scans but not resident
Maybe I will turn it off...
The problem isn't the dimming of the desktop, but I will disable it just in case has something to do. My problem is that, before UAC popup appears the netbook entire screen turns off for a second as if the resolution was changing. This doesn't happen on my desktop.
I get the same when switching users or logging off. But in this case it occurs on my desktop too.