How can I launch an instance of an application using Python?

While the Popen answers are reasonable for the general case, I would recommend win32api for this specific case, if you want to do something useful with it:

It goes something like this:

from win32com.client import Dispatch
xl = Dispatch('Excel.Application')
wb = xl.Workbooks.Open('C:\\Documents and Settings\\GradeBook.xls')
xl.Visible = True    # optional: if you want to see the spreadsheet

Taken from a mailing list post but there are plenty of examples around.

or

os.system("start excel.exe <path/to/file>")

(presuming it's in the path, and you're on windows)

and also on Windows, just start <filename> works, too - if it's an associated extension already (as xls would be)