harddrive space missing/ scan disk crashes computer

hi,

i've been having virus/trojan problems of late, but this
is the first time half of my hard drive is missing.
actually the computer says it's in use but as i've just
rebooted to factory settings and have not had a chance to
fill it up yet......so i don't know what's going on.

also,in trying to figure this out by using scandisk, i
have had the computer freeze up not less 10 times in the
last couple hours.

any help would be greatly appreciated.
thank you