SP1 breaks Explorer

Twice now I have tried to install SP1 on a x86 netbook. Both times the installation went smoothly, and both times afterward Windows booted up to pointer on a black screen.

I hit Ctrl-Alt-Del to get Task Manager, and from there opened Explorer. Windows then says it is setting up the desktop for Microsoft Windows, but it opens to the User Account I have set up. At that point I check Processes in Task Manager, to find that only four are running, whereas usually there are maybe twenty or more.

Somehow SP1 is hosing the boot process. I'm come across mention of this problem elsewhere, but no solution. I guess my ace in a hole is a factory restore, but I'd rather not. Any ideas? Thanks.

Even SFC didn't help, so I went ahead with the factory restore. SP1 is now installed.

and is it solved? Otherwise i recommended to try in Safe mode with networking and go to Revo Uninstaller Pro - Uninstall Software, Remove Programs easily, Forced Uninstall, Leftovers Uninstaller and see if you can delete the whole SP 1 included folder and registry paths. and install again.
Also you mentioned that it opens users account instead explorer.exe right? Are you logged in own user or temp user?

Yes, solved. I hadn't thought of safe mode, but maybe it's better the machine was taken back to factory anyway. I had very little invested in the installation, so little loss. It went to the main user account, after a strange intro box that looked like it was setting up a new and spurious account, but explorer and other processes were never started.