A couple of days ago, I was browsing, and downloading something from Usenet, when my iMac all of a sudden froze up, grinding to an unbearable slowness. After trying to tolerate what was happening for 5 minutes, hoping it would pass, I finally gave up, and restarted the computer manually. To my horror, for about a day, I was plagued with the interminable locked up gray Apple startup screen, but thankfully the next morning, left it on from a restart to find later that the computer had managed to boot up to a login screen, everything seemingly fixed.
That was background. Now I have a different problem: My iMac is subjecting me to intermittent freezing episodes; I will be writing, or browsing in peace for a few minutes, before everything locks up, save for perhaps only the cursor. When the hanging is finished, the computer then resumes the missed operations (typing, etc.) in a hurry, as if one were watching a fast-forwarded video. Each hanging episode lasts from anywhere from 5 seconds to upwards of a minute; moreover, the system pops a window telling me that I don't have enough disk space for memory, though Finder consistently tells me I have upwards of 30GB worth of space (on a 250 GB HD). There's also another curiosity: When I tried using Software Update to update everything — trying to fix this problem — upon Restart it told it couldn't install a recent Security Update because "the source media [you?] are installing from is damaged."
What's my causing my problems? How can I fix them? Unlike a previous related question, I don't think this has anything to do with Youtube or my browser; closing near every window, including Chrome, has no solved the problem, or mitigated it in any way. I was also wondering could it perhaps do with something I downloaded -- could Trojans somehow associated with the movie I was downloading have anything to do with this problem? I'd really appreciate some help on this; this freezing has rendered a significant part of my workflow virtually unusable, and I don't consider myself very computer savvy.