iMac freezes up

I got my iMac in2008. I've NEVER had a problem, until now! The iMac has begun to freeze up every time that I use it. I get the SBOD after about an hours use. If I start using iTunes, it begins even quicker. I can't go into sleep mode, or power off using the button in the back. I have to unplug it every time. This began a couple of weeks ago when I updated iTunes. Sometimes, when I plug it back up and restart, it goes right back to where it was when it quit! PLEASE HELP!!!!!!!!!!

G'day and welcome.

What operating system? Helps if you provide this. How full is the hard drive? If running OS X.5 or X.6 boot from the system disc, hold own 'C' after the chime, leave Installer and go to utilities > Disk Utiltiy and run Repair Disk.

For later see this:-

http://support.apple.com/kb/HT4718

Follow what is posted above as that is some good advice, also can you get a s screen shot of your Activity Monitor clicking in the Real Memory column with the triangle pointing down. You might have to click the column twice to get to it.

I thank y'all for the quick responses and the help! If it stays on long enough to try the suggestions I'll be very happy.