sometimes the task manager won't show *everything* thats running in the b/g (i.e. a virus or trojan). about 2 yrs ago, i had the same problem... couldnt get scandisk or defrag to run. my computer wouldnt shutdown either. it was just freeze on the "windows is shutting down" screen, but never fully go down. had to manually power off and back on. turns out had a virus that snuck itself into nv ram every time the computer was restarted, so even if u deleted the virus, then rebooted, it was back again constantly. had to clear nv ram in bios and clear every instance of it on harddrive (about 100 of them).
this prolley isnt yer problem, but worth checking into anyway. just go to start, run and type in msconfig. go to the startup tab and look at all of the items that are set to run at startup. uncheck anything that looks suspicious, reboot, and see if its rechecked.
dss