Vista hangs many times just after boot

Hi all !
I am using Vista beta 2 on a notebook HP Pavilion DV4000, with 1 GB RAM, and
I am facing a strange behavior:

When it is turned off for a while and I boot it, after logon, if I try to
utilize Windows Explorer, Internet Explorer or some other softwares in it, it
hangs the software and everything else hangs. There is no way at all to close
or kill the hanging process. Any other software cannot be loaded without
hanging. I cannot kill the hanging process using Task Manager, and, if I
utilize the command TASKKILL with all possible options, it says that the
process is killed, but it remains in the Task Manager.

In this situation, I have to turn off and turn on the computer because
nothing else works (restart does not work), and this behavior repeats three
or four times, since without a clear reason it works fine after that, and
remains stable all day long.

But if I turn it off, maintaing it turned off for a while, and turn it on
again, the problem occurs again.

While this problem is happening I did many different tries, as booting
without network connected, with network connected, waiting for some time
before loging on, using only bateries, using AC, etc.

Important: I was utilizing XP before without any problem at all.

Does anybory know what could be going on? Or have suggestions for a more
precise diagnostic?

Thanks for the help.

Duarte.