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Thank you all so very much for your detailed and informative answers. They are all very useful and your help is much appreciated. Things have been hectic for me and I haven't had time to actually put your links to good use yet but I certainly will do right now.
ZebUK - I did go in to task manager but I had over 40 processes running - a lot of which looked suspect and according to a google search , many of them were associated with trojan/malicious spyware. I was running at 100% cpu usage which is worrying because I was offline and I have never been anywhere near that in the past. I don't think it can be down to how long I have my pc running though Ethel because as I say , even when I have very little running , I am still at 100 %. I'm going to try to rectify the problem now though - I just wanted to pop in to thank everyone for their help.
Thanks again everyone x