first you have to find the folder that 2.1 is located in. On my computer (Vista) it is located here: (if your on a mac it will be someplace else)
C:\Users\me\AppData\Roaming\Apple Computer\iTunes\iPhone Software Updates
me is my user name on my pc - on yours it will be your user name. erase:
iPhone1,2_2.1_5F136_Restore.ipsw
and the next time you restore with itunes it will redownload the file.