I have an ipod touch 4th generation.I had download ios5 but it got cancelled.Now when i redownload it, it says error. What should i do.

If this does not work, try to delete the ipod software file located in C:[username]\Application Data\Apple Computer\iTunes\iPod Software Updates to make iTunes download a new copy when you connect in recovery mode. Maybe the file stored on your PC is damaged.

Rename, move, or delete the iOS software file (.ipsw)

iTunes uses "ipsw" files to restore your iPod or iPhone. If those files are unusable, then try deleting them, renaming them, or moving the file to a different directory. This will cause iTunes to download a new copy of the IPSW. When a restore issue is specific to a user, it is likely due to an unusable .ipsw file. If removing the .ipsw file does not resolve a user-specific restore issue, then the issue is caused by other user-specific security software settings or iTunes preferences. Creating a new user will also cause new .ipsw restore files to be downloaded in the new user. You can find the ".ipsw" files in these locations:

Mac OS X

iPhone	~/Library/iTunes/iPhone Software Updates
iPad	~/Library/iTunes/iPad Software Updates
iPod touch	~/Library/iTunes/iPod Software Updates
Note: The tilde "~" represents your Home directory.

Windows XP

iPhone	C:\Documents and Settings\[username]\Application Data\Apple Computer\iTunes\iPhone Software Updates
iPad	C:\Documents and Settings\[username]\Application Data\Apple Computer\iTunes\iPad Software Updates
iPod touch	C:\Documents and Settings\[username]\Application Data\Apple Computer\iTunes\iPod Software Updates
Note: If you don't see the Application Data folder, you may need to show hidden files.

Windows 7 and Vista

iPhone	C:\Users\[username]\AppData\Roaming\Apple Computer\iTunes\iPhone Software Updates
iPad	C:\Users\[username]\AppData\Roaming\Apple Computer\iTunes\iPad Software Updates
iPod touch	C:\Users\[username]\AppData\Roaming\Apple Computer\iTunes\iPod Software Updates
Note: If you don't see the AppData folder, you may need to show hidden files.

copied from http://support.apple.com/kb/TS3694