Windows 7 - Desktop icon and start menu not loading - Win7 Ultimate

Hi everyone.
I installed Win 7 Ultimate recently on top of my Vista Home 64bit, Vaio laptop. Setup archive was downloaded from microsoft website.

Installation was fine, first start up was fine. But after second reboot my desktop stopped loading, its was just a blue screen with sign at the right bottom corner - Windows 7 Ultimate Build 7010 or something. Couple times I pressed ctrl-alt-del and started dwm.exe and explorer.exe manually once again (although these 2 processed were already in the list) and was managed to run desktop.

Yesterday I installed some IE8 and Office updates and changed theme to Aero, and now after reboot Im getting same thing with black screen, but dwm.exe and explorer.exe does not help anymore (again, they present in the list of processes), no desktop, no start menu.

I use firefox with noscript plugin, user account and antivirus software, so I dont have any virus issues or something. This trouble began right after installation is over.

What can It be? How do I solve it?
Thanks, Alex

Re-installed windows7 again, first boot - OK, than restart or log of - desktop not loading again. So I have to start explorer.exe manually.. Any ideas?