[theory]

Hi

I was scaning my hard disk and I was told
by fsck that I have a missing thread. This
was on my boot HD. I tried using Disk
First Aid it gave me the same error but
it couldn't fix it.

How serious is this problem. I know I can
just format the HD to fix it but I can't
right now (well I am kind of very busy next three weeks and don't care if the
iMac's HD dies on me)

[Rainy Day]

I don't know for certain what it's trying to tell you, but it might be a corrupted directory. If so, DiskWarrior will likely fix it. Worth having around in any case.

Otherwise, perhaps this page will be of interest: Mac OS X: About Using Disk Utility and fsck for File System Maintenance

Best of luck.