Help. viruses on computer wont go away?

i have microsft secturity essential. i have done a ful scan it is telling me to do it again every so often it tells me i have a virus and then changes in a second from 1 to 2 to 3 within a second of how many viruses. this has just happened today and i havent downloaded anythnig or done anything. it keeps popping up a million times and driving me crazy i keep pressing clean comptuer or remove and more and more come up. please help what do i do? i am doing my second full scan but i keep doing it and it isnt helping