Okay, not sure if this is of any use to anyone else, but it helped me...

I downloaded the symantec file. Ran through the instructions (ie, deactivating System Restore, opening up commandprompt, doing that weird chktrust -i fileblast thing or whatever and all that crap) and got those errors I was talking about. I checked what file it was stopping at when it got the error (by this time I was doing everything in Safe Mode, btw, following Symantec's instructions), and found it was inside my Temp folder. Funtime! I deleted every last one of my Temporary Internet Files until I narrowed myself down to only having one file remaining, this was under Temp - Content.IE5, in one of those many jumbled folders. I couldn't rename, delete, access properties or anything, but it was still there. I ended up trying something else - opened up command prompt, moved into that directory and ran <b>dir/w</b>, and found only that file. I ran <b>del *.*</b> and it was gone. After that problem, the scan ran smoothly.

The scan took bloody ages, even after deleting all that crap. I can't find all the files that it's going through in my Local Settings folders at all, but after a few hours it finished it's scan and had deleted one file from my computer. I then continued with Symantec's instructions and ran msconfig and removed Safe Mode, restarted and logged into my normal account, and at 6am this morning I started the scan again, as per instructions. The scan was still running when I left for work. Once I go back, I have to ensure that the patch is installed and then I have to reactivate System Restore.

I'll let you know how I get on =P