Application Non Responding (Vista)

I am running Vista Home Premium SP2 and few days ago I added a new
graphic board. I also installed some new software and my system became
very slow. The system worked well and fast for just 30-40 seconds and
then freezed for 2-3 minutes with the windows of the application
saying "Non responding" ("Non risponde" in italian)

I was not able to understand the cause of the problem, so I decided to
reinstall from scratch.

I now have a system where Vista is just reinstalled with all the
current updates. I didn't see any problem related to the updates
installation.
But still I see from time to time some windows "Non Respondig"

On the system I have installed the drivers of the new graphic board. I
don't see any error or conflict of the boards. I had tried to run both
boards with two videos: it was funny and it worked.
Now I have disabled the original board that is on the mother board,
but with this board enabled or not, it doesn't make any difference.

On the system I have now installed only Firefox, Kaspersky 9 and Adobe
Reader. So it should be a very light system. Still I have some freezes
that became more severe while I was installing these few softwares or
while I was scanning the system for viruses.

I have just 2 Gb of RAM, but the system was working well before the
installation of the new graphic board, and at that time it was really
overlaoded with application, including the same Kaspersky 9, Firefox
and Adobe Reader plus many others.

How can I check if it is a problem of conflict of Graphic board, RAM,
conflict between some Windows updates and my applications or failure
of some other component?

Could somebody advise me on some diagnostic tool or process to follow?

Thanks in advance,
Silvio