Most probably a badly-coded indicator or EA.
Try the following:
When launching MT4 do you have a chart being loaded by default with an EA and/or Indicators loaded on it?
Perhaps configure your chart exactly as you would like it, then save that as a template.
Then when you close MT4 down, close the charts first so there are none open.
When you launch MT4, then simply open whatever currency chart you want opened, and then apply the template you saved above.
Your EA and/or indicators are probably trying to process your charts data at the same time the system is loaded.
Just a suggestion, but I previously had a similar issue, and this resolved the problem for me.