System freezes, apps not responding...probably HD failure

basics:

iMac 27"
Mac OS X
10.6.8
3.06 GHz intel core 2 duo
4 GB Ram
3 MB l2 cache
Graphics: ATI Radeon HD 256 MB

HD:
start up disk/system disk 1 partition SATA 1 TB
write status: R/W
S.M.A.R.T. status: verified
partition map scheme: GUID partition table
connection type: internal


now the problem is that everything was fine and normal when suddenly while i was working everything freezes and the mouse keep loading, finder and all apps freezing -
first time to see errors on mac - not even a force quit, had to restart manually, everything returned back to normal and after 5 mins freezes again, restarted and unplug all network connections, freezes again....restarted for 3rd time, and by the time the apple icon appears (loading screen)...i heard such a sweeeeet sound i heard 5 years ago when my old HD failed, a sound like a metal trqap....and the loading screen kept loading forever, about 45 mins, i restarted many times but the system won't start up,
good news is that i haven't seen any flashing question mark yet - indicates that there is no HD connected or failed - it is just loading and loading....

i haven't installed anything new, i don't use bootcamp or other related stuff...i just keep opening my system for days and long periods - that was the purpose for buying mac,
am working on video production and rendering all the time

the machine was very hot, so i shut it off for 24 hrs

today i reopened it again for a check, it took a little bit long time on loading at start up but finally opened and everything was fine and normal, so i backed up everything on an external HD, and checked the system performance for 1 hour and everything was normal again

i tried to go to disk utility to see if i can check for a bad sector or something, i found the repair button is unaccessible so i tried to verify disc permissions- note that i don't have any problem with permissions, it is just freeze - but i couldn't do anything there

thank god i haven't HD failure till now, but in the future, how to avoid this thing?
how to check for bad sectors?
if i made repair from installation disc, would this affect the data?!
i got a 1 TB external HD drive for back up, can the time machine help me here?! can it restore everything automatically later (data, settings, preferences, etc.....)?!
could it be a malware?!

HELP plz

update: i made a disk repair, the S.M.A.R.T. status: verified
i made also repair permissions and completed successfully...
yet i don't know how to know if there bad sectors or not, how to save it from incoming failure and to avoid data loss!

The good news is that you've been making backups. Keep doing that. It does sound like the hard drive may be intermittent and perhaps getting ready to fail.

The heating up part is also worrisome because the hard drive in your machine has a fan with heat sensor to keep it cool. If your iMac is covered by Apple care, it might be a good idea to make an appointment and have them check it out. Changing out a hard drive in an iMac is not something you want to do yourself unless you absolutely have to.