I have created a few site workflows that have been schedule to run at various times throughout the day.  I've recently noticed that when the job runs and updates the fields in a list, it updates it so that the "modified by" is under my name.   Normally this isn't a problem, but as I'm leaving this specific company soon, I'm thinking this will become an issue when they disable my Active Directory account.  I've read some postings around how to run the job under the workflow owner, but I want the reverse.  How do I set the site workflows so that it will run under a system user?

Thanks everyone for your help so far.