when you start up in safe mode using F8 as soon as the desktop loads it says 'safe mode' in the corners of the screen. if all is well here then try this:
click 'start/run' and type 'msconfig' and click ok. next click the 'start up' tab, make a note of what is listed and then untick everything and restart normally. ignore the warning and tick dont show again.
if everything is ok now go back and tick one at a time and restart each time to try and find the culpret maybe. all i have starting with windows is my AV and firewall.