Hi,
What Win do you have ?
Are all Windowsupdates applied ? CHECK!!
please supply the full path/folder name of the 12GB-folder
(could be temp. unpacked files by avast that didn't get deleted after a crash)
have you tried disk-cleanup & defrag in SafeMode (F8-Boot) ?
or a boot-time scan (if you have Win NT/2000/XP )?
have you tried an Onlinescan with Trend (see below) or
www.ravantivirus.com ?