FAQ: Why do I have a red or yellow bar over my sequence, and how do I render previews?

Question

The first thing to know is that a yellow bar isn't a sign of a problem (nor is a red bar, necessarily).
See this article for details:
Red, yellow, and green render bars and what they mean
... and this movie for a somewhat less detailed explanation:
Why is there a red or yellow bar in my sequence?
Here's the summary:
green: This segment of the sequence has a rendered  preview file associated with it. Playback will play using the rendered  preview file. Playback at full quality is certain to be in real time.
yellow: This segment of the sequence does not have a  rendered preview file associated with it. Playback will play by  rendering each frame just before the CTI reaches it. Playback at full  quality will probably be in real time (but it might not be).
red: This segment of the sequence does not have a  rendered preview file associated with it. Playback will play by  rendering each frame just before the CTI reaches it. Playback at full  quality will probably not be in real time (but it might be).
none: This segment of the sequence does not have a  rendered preview file associated with it, but the codec of the source  media is simple enough that it can essentially be treated as its own  preview file. Playback will play directly from the original source media  file. Playback at full quality is certain to be in real time. This only  occurs for a few codecs (including DV and DVCPRO).
Note the uses of the word probably above. The colors aren’t a  promise. They’re a guess based on some rather simple criteria. If you  have a fast computer, then a lot of things marked with red may play back  in real time; if you have a slow computer, then some things marked with  yellow may need to be rendered to preview files before the segment can  be played in real time.

Answer 1

This is not the work of a virus, or any other kind of malware. What it sounds like is a very badly corrupt hard drive causing all manner of corruption in your various system components and applications. You probably need to erase your hard drive and reinstall the system from scratch, as ds store has said.
If you actually had malware, Sophos would find it. If Sophos found nothing, there was almost certainly nothing to find. If something like Sophos ever should find malware, unless the malware has "OSX" or "MacOS" in the name, it probably isn't Mac malware and thus isn't something you need to worry about (other than not passing it on to other people).
MacScan is junk. It serves no useful purpose. See:
MacScan disappoints