Not sure if anyone has started a thread about this yet but if you have Windows 7 DO NOT INSTALL SERVICE PACK 1. It has been causing a lot of people fatal errors and they have to start their machines from scratch. Just a heads up!
My problem was having mapped network drives, and the file server was offline when I tried to boot. Windows responds by looking very much like the SP1 pack failed, or you've acquired a virus and can't boot or log in. The solution was to unplug the network from the Win7 machine until it booted, then replug the network cable back in. I was almost at the point of reinstalling Windoze myself, before looking at this possibility as a last resort.
As a result, I no longer use mapped-network drives. IMHO, Windoze can barely be called an operating system because there are lots of boundary conditions like this on which it fails. Linux/Unix ftw!
First time I installed, Couldn't get windows to start got a blue screen everytime it got to the welcome screen, I had to do a fresh install of windows but now my computers fine and im running SP1.. strange