Any Guidance for Fixing a Sluggish iMac. New Mac User, would appreciate the help! :)

I'm running an iMac that is roughly 1 1/2 years old. I don't have the exact specs to hand, but the next time i'm on it i'll put the details on here, though O can tell you that it has 4GB of RAM and a Intel Duo processor, and has only about 40% of its hard drive full (200 out of 500GB).

Over the past month or so, my iMac has been very sluggish, and its developed a few bad quirks that just won't go away. The computer will "freeze" in that the mouse and keyboard will be unresponsive, but any background applications will still run (i.e. video still plays, background picture changes). I can shake the mouse, or even unplug it and plug it in again, which solves the problem for all of 10 seconds.

Overall the Mac has become painfully slow, I use the firefox browser, which has become a lot more sluggish over the past few months. Pages take a while to load, which is abnormal, they usually snap on, but the time it takes for them to load is noticeable. Applications take a long time to load, usually a few minutes, and at sluggish when they are used, VLC for instance, would usually take a few seconds to load a movie, now it takes a minute or so.

I've tried running all the standard options in Onyx, which I've been told can help with maintenance (repair permissions and all that). I've cut down any superfluous applications, and deleted any unimportant data. But the computer still has a nasty habit of being slow. I'm running a few NTFS USB External hard drives, that could be a problem i guess, I do have an external piece of software called ntfs-3g, which could be a potential cause i guess. But this computer never had any problems with this before.

I also have a changing background picture, which changes every 5 seconds, and runs through roughly 170 HD photos, that could also be the cause? But i'm really not sure at all.

I tried calling the Mac helpline, but they wanted to charge me, and i'm really not interested in being charged for being told how to keep a computer running optimally. I'm originally a PC user, and i'm a bit anti-apple for all these extra charges they try and lump on you for owning their products, though i'll gladly admit that they do produce good machines.

Do any of you have an advice or guidance that they can give me for reviving my iMac, I'm still quite new to this system, and i'd appreciate the help

I would turn off the background picture change for now.

Try disconnecting the external drives to see if that improved things

Otherwise I assume you are using 10.6 Snow leopard or Leopard
Download the latest Combo update for your system

Mac OS X v10.6.7 Update Combo
Mac OS X 10.5.8 Combo Update

It may seem odd to update your system if it already up to date, but a lot of the time the combo update refreshes a system that has undergone a series of incremental OS updates.

My guess would be that turning off the constantly changing desktop picture -- and removing all files that are sitting on the desktop and filing them properly away inside the hard drive (just a guess but I suspect this is the root of the issue) and a good restart will work wonders.