Everything is running slow

My computer has suddenly started running extremely slow no matter what I do,
internet, OE or word. I had trouble with my Norton programme not responding
& the computer was a bit slow then but I disabled it & downloaded a free
trial of another programme from a free disk which came with a magazine which
at one point said it had changed the AUTOEXEC.BAT file. Since then my
computer has run at a snails pace. I have got rid of that programme now &
downloaded AVG which is working well but the computer is still so painfully
slow to to point that sometimes programmes stop responding. Can someone tell
me what to do to get it back to normal speed please?