VBA has a CurDir keyword that will return the "current directory" as stored in Excel. I'm not sure all the things that affect the current directory, but definitely opening or saving a workbook will change it.
MyWorkbook.SaveAs CurDir & Application.PathSeparator & "MySavedWorkbook.xls"
This assumes that the sheet you want to save has never been saved and you want to define the file name in code.