You really need to post everything that you were doing and the format you are using.
Can't really help too much with out a bit of info but here goes....
ok first I and a few others stress this alot...DO NOT USE MP3'S IN AFTER EFFECTS this could be one problem and depending on what you are doing and how you do it may not become a problem until the final export. Secondly why the hell are you exporting to 12 mb (what you said in other thread), Im guessing that the vid is around 4mins, so this is gonna realistically be too small for any decent quality, incidently exporting to quick time doesn't really cut (if it is just 6 secs of fx then prob ok but again depends on what you are doing!) it by this I mean more info on the actual settings like frame rates? After effects is well known for getting screwy with you if you are a. using non standard frame rate (for example 60fps...wont go into interlacing but trust me things can go wrong here) and b. if it is interpreting ata different frame rate, generally it wont crash your machine though BUT as you can see all this info would help a little. This might not be clear but I'm talking during edit and export.
There is of course that you have been heavy on the time and distort effects and your machine can't handle it.
I'm starting to babble and not make much sense but basically
What version of After effects do you have?
What is the format of the imported footage and audio? (also would like to know frame rate)
What format are you exporting to?
If quick time what settings are you using, e.g compression type, frame rate, depths and channels etc.
Where are you exporting to?
If an external hard drive what format is the drive and have you change the drive letter or anything like that recently?
What spec is your comp?
What is the exact error you get?
Does the effect envolve any third party plugins or anything worth a mention like that.
Have you made any changes to your preferences?
It may be footage corruption, I can't remember the specifics but maybe a year or something ago I had a similar problem and was able to isolate it to a few seconds of footage, once that footage was deleted the problem was gone. This has happened to other people I know using the noise effect and was solved in different ways.