2008 iMac 24" turned extremely slow and unresponsive

[mrsk]

I've had a few problems where the system would come to a halt for a second or so for some time now, especiallz while viewing EyeTV, the video would pause for a second every half minute and then continue playing. Now since today I cannot do anything at all with the Mac anymore. Listening to music on iTunes would get me longer pauses than actual music playback (around a 10 second pause for every 5 seconds of music playing). Simple tasks like starting the activity monitor took well over 5 minutes. It feels as if my system is pushing a constant 100% CPU load. Looking at the activity monitor however, I only have a maximum of 5% CPU load, only very little HDD activity and around 1.2GB (of 2GB total) RAM listed as free (and another 500ish as inactive).
I'd really hate to have to have it checked at a local Mac dealer, so maybe there are some things I could try myself first?

Thanks a lot in advance

Edit:
Forgot to mention... the same problems occur under Windows (7, RC1 Build 7100). Disk Utility doesn't find any errors on the HDD and repairing permissions will find a lot of broken permissions, but it doesn't actually repair any of them. Did the whole repair procedure 3 times over, it still lists the same errors as before.

 

[suburbia]

Check this out, hopefully this will set things right:

http://forums.dpreview.com/forums/read.asp?forum=1017&message=28203408

Good luck

 

[mrsk]

I already use smcFanControl to control the heat. Thanks for the suggestion, but I don't think that's the cause. I do however get the feeling that my HDD might be dying again