A couple of times my defrag would keep restarting when I had my cat5 ethernet cable plugged in...I unplugged it from the comp and defrag finished normally....it doesnt always do that but it could be that.
Also, not everything that is running will show in "ctrl-alt-delete"
I would POSSIBLY consider running "adaware 6"..the free spyware cleaner...
www.lavasoftusa.com just in case there is junk like trojan horse programs running that are causing activity and making it restart scandisk.
also, for safe mode....I think you can just press f5 when booting...press it over and over, dont press and hold down....(or press f8 and then choose "safe mode..either way works)
when you reboot from safe mode it will start notmally next time.