Downloaded file corruption

Question

iBook G4/1.42, 512MB RAM, 10.4.9 on combo update. Machine running absolutely normal before. Been on 10.4.9 for some weeks. No problems whatsoever. Unaltered OS X apps in use. Regularly disk permissioned, cache cleaned, sudo periodic daily/weekly/monthly script in terminal. 21gb free space.
2 days ago, from nowhere, PDFs, image files, MS Word docs and MS Excel docs that come into Mail are seen and detected. It comes up with the correct preview, has the correct size etc. But they won't open. Only Word doc that has opened has been one with no images. So the problem is to do with images, it seems to me.
I thought this may be a Mail.app problem, but I downloaded an image file from my server (via FTP program) and file still corrupt - so it suggests a local problem on my iBook or something seriously wong with the server (other users on PC and Mac report no problems). I also checked the server (Windows I think) for hidden files, nothing strange there.
Image files, pdfs, word and excel docs on the iBook already are fine.
So maybe you can see why I am confused?
I then checked Console and discovered a world of logs that I only part understand - error messages seem to be 'corrupt jpeg data-premature end of data' and 'extraneous bytes before marker'.
The error message on MS Excel files are: "the file may be read-only, or you may be trying to access a read-only location. Or, the server the document is stored on may not be responding". Needless to say the xl file is NOT read-only, and the mail service is POP not IMAP.
The MS Word message is: Word cannot open the document. Try one or more of the following: *On the File menu, click Open to open the document. * Make sure the document is a valid Word document. Needless to say it is a valid word document.
Images either don't open in preview, or the window just shows some scrambled stuff at the top.
In all cases, have attempted to open files from Mail or from the desktop.
Although I'm British I travel a bit and this problem has started in China. But I travel here frequently, never had this problem. I'm wondering whether their cyber-curtain is corrupting files.
I sent myself an image from the laptop's hard drive, the e-mail back was corrupted.
Word, XL and image files on the iBook open as normal.
I forwarded an incoming mail with an XL attachment to the UK, they can open it (on Mac and PC). I asked them to zip it and return it. It didn't open.
I've repaired permissions, there were no unusual messages. Have had no power outages or other disk corrupting events.
I looked in HOME/LIBRARY/PERMISSIONS and found 2 lines for Word and Excel. I moved them to desktop and re-opened xl and word. The programs work fine, they made new permissions files. But Mail attachments don't open.
Mail Downloads file has permissions that say I can READ/WRITE.
Any ideas anyone, please?
Sorry to go on so long, have tried to give as much info as possible.

Answer 1

Hi, this is what happened. You'll have to excuse my ignorance, although i thought I knew my way around Macs, actually I have never done this single user mode before. So, I clearly know less than I think! Here goes:
started with apple/s on start-up; got black screen with white words.
Typed in fsck -y. It came back with message that HFS is journaled, had to be fsck -f. (incidentally, in OS X, checking disk in DU, the dialogue says S.M.A.R.T is verified).
Did a fsck -f. Dialogue looks like DU Verify Disk actions. Result was Disk OK, no errors.
Followed instructions for reboot, sh /etc/rc. Dialogue I noted includes IPv6 initialised, default to accept; error 01, error number 65 for MSNS platform send UDP failed to send packet. (this seems like my Firewall settings where all options in 'advanced' pop-up are ticked. I mention this coz the error messages in Console suggest some bytes are either missing or have been added to the files.
Loading in single user mode stops after working bonjour.
re-enter sh /etc/rc, receive message boot cache control - cache failed to initialise. No other obvious failure messages. loading still stops at working bonjour.
re-entering of sh /etc/rc gets a long list of bootstrap commands.
no progress thereafter, just a repeat of the localhost prompt.
In the end, powered down and started, OS X starts as normal.
screen never moved beyond black background white words (similar to MS-DOS prompt in Windows?); is it supposed to load a basic OS X?
What next?!