Here's the thing, I read this solution on another thread & tried it. Now everythings gone to s**t!!! I did it exactly step by step.

Some problems that have arisen since:-

1. I can't actually do anything on the Finder (can't move icons, can't do anything)

2. The computer now is slow as ****.
3. Programs aren't doing what there supposed to.

Can anyone help me here? I'm starting to worry I've completely messed up the comp?

Here is the original solution:-

Download the following:
1) Pacifist
2) Applejack
3) The Mac OS X 10.4.9 combi updater for your system (Intel or PPC)

Install Pacifist and Applejack. Run Pacifist. Mount the 10.4.9 combi disk image, and drag the .pkg file to the Pacifist window.

In Pacifist, open the folder System/Library/Extensions and select the files IOUSBFamily.kext and IOUSBMassStorageClass.kext . Click Install. Reboot.

Reboot again and hold down Command-S when you hear the chimes (but first, see note 4 below). After all the text stops, type 'applejack' at the command prompt. Then enter '3' for 'cleanup cache files'. This will clean up the system cache. I also then cleaned up my user cache files ('/User/xxxx'), and then on returning to the main applejack menu, also selected '5' to clean up swapfiles (see note 3 below).

Once you have done all this, exit applejack and reboot (hit enter at the main menu, then select 'r' for reboot). Your USB drive should now function correctly again.

1) Clearing the caches using applejack seems to be essential. Simple replacing the kernel extensions did not work for me until I cleared the caches.
2) You probably don't need to clear out the user cache or the swapfiles, but I did these, so I'm mentioning them in case they turn out to be important.
3) This basically rolls back the USB external drive support of your Mac to 10.4.9 but keeps all other functionality of the 10.4.10 update. So, the people with USB drives or peripherals only supported by 10.4.10 (e.g. the TomTom whatever-they-ares) will probably not wish to use this workaround.
4) You might have to unplug external devices when booting into single user mode, or else (like me) you might find you get persistent timeout error messages from one of your USB peripherals scrolling up the screen. You might also find your external USB keyboard doesn't work (on a Macbook). If in doubt, unplug everything and boot into single user mode on the bare machine.

Tech comments: My external drive case is a generic USB2 model, apparently constructed by Acer: Under Mac OS 10.4.10 it is detected by USB Bus Probe as:
Device VendorID/ProductID: 0x0402/0x5621 (Acer Labs Inc.)
Device Version Number: 0x0103
It is also detected by System Profiler, and listed under "Hardware->USB" as "USB 2.0 Storage Device". The problem therefore appears to be one of not mounting, rather than not being detected.