How to get currently logged-in user in workflow

        /// <summary>

        /// Get current user in workflow sharepoint context

        /// </summary>

        /// <param name="workflowProperties"></param>

        /// <returns></returns>

        public static SPUser GetCurrentUserInWorkflow(Microsoft.SharePoint.Workflow.SPWorkflowActivationProperties workflowProperties)

        {

            string ModifiedbyUserName = Convert.ToString(workflowProperties.Item.GetFormattedValue("Modified By"));

            string[] strChar = { "ID=" };

            string[] strChars = ModifiedbyUserName.Split(strChar, StringSplitOptions.None);

            Int32 iUserID = Convert.ToInt32(strChars[1].Substring(0, strChars[1].IndexOf("\"")));

            SPUserCollection usersInSite = workflowProperties.Web.SiteUsers;

            int countUsers = usersInSite.Count;

            for (int iCnt = 0; iCnt <= countUsers - 1; iCnt++)

            {

                if (usersInSite[iCnt].ID == iUserID)

                {

                    return workflowProperties.Web.SiteUsers[iCnt];

                }

            }

            return null;

        }

Refer from http://www.sharepointkings.com/2008/08/how-to-get-currently-logged-in-user-in.html