Due to virus issue the drive icon has been changed to .dat file icon( Unkown file icon)

Run this onlien virus scanner
http://housecall.trendmicro.com/

Install, update and scan under safemode with SAS
http://www.superantispyware.com/

also, goto Explorer>Tools>Folder Options>View and choose Show Hidden Files
apply and check under C: drive (your system drive)
delete any autorun.inf file if present here
post back the results

if norton had already deleted the virus, chances are the effects on the OS can be permanent. I would suggest "sfc /scannow" to restore file associations or maybe a full repair for any other "yet-undiscovered" aftereffects.

Here is a great collection of REG files to restore the default actions for numerous filetypes.  Get the one for Drive Associatios and run it. http://www.dougknox.com/xp/file_assoc.htm

First i shifted the data into another computer and I Scan the antivirus and antispyware it shows some Adware and i remove that files . I formated all the drives in main computer and i reinstalled the OS and then i restored all the files from that computer to orignial computer after 2days i get the same issue .plz give me the solution.

what kind of data are you backing up and restoring?
have you run SAS and antivirus scans on your external drive which contains this data?

SOMETHING is altering the associations in Windows; meaning, the part of the registry that has the info on what to do for each type of thing (i.e. drives, EXE files, etc).  If it recurrs again, we'll really need to dig in; but, my first post is to a collection of reg files you just double click on to fix varous problems.
"Here is a great collection of REG files to restore the default actions for numerous filetypes.  Get the one for Drive Associatios and run it. http://www.dougknox.com/xp/file_assoc.htm "