Thanks Morgwen.

I posted my problem on the Apple Discussion Boards and through some advice I recieved there plus some things I had done before I have managed to solve the problem. Although I suspect it might come back.

Strangely enough, it seems to be caused by Font Book, specifically one of Font Book's preferences called '.com.apple.ATS.plist'. So I deleted this and restarted my Mac and all seems ok so far. I also deleted Lime Wire before I did that, which had no effect other than dramatically limiting what I could do with my Mac.

I still can't work out Lime Wire's part in my problem... why my Mac worked fine while it was running... and why it apparantly counter-acted whatever problem the Font Book preference seemed to be causing.

As much as my Mac's working fine just now, I am still not content with the fact that I have experienced this problem twice. So in the mean time I will be keeping Lime Wire off my Mac while I try to get to the source of the problem.

It all seems very strange to me.

However, I take what you suggest about Virus' very serious... as that was my initial suspicion and worry.

You suggested 'downloading' a virus scanner. Could you suggest one as I never realised you could download them freely.