Well I have absolutely no idea what caused it. I did a clear out but nothing changed, then - miraculously - it was all okay by about midnight.
I had the same problem a couple of weeks ago - everything was going really slow and it drove me crazy. I also found out that Dan and Lauras computers were also on a go slow (this was after I had done the defrag thingy) This also happened again for a few nights after about the same time 8 ish at night and I think it was our server and not the actual computers. We are with Wanadoo - who are you with?