IE8 opens, in the task manager, instances for all separate tabs, etc that you are using. This is normal and is the way IE8 is meant to function. In your case, however, As you are not actually using the computer, I would suggest the possibility of Malware. The only other explanation could be that you have a resident program which is scheduled to search for updates or similar, periodically. ´Such a function could produce the result, but it is unlikely.
Do a full malware scan, or download and run another free anti malware program to see if you get any bad hits.