give him a break, it is the internetz.
anywho.
you can solve this with the 'ren' command, known as Rename
ren *.mod *.mpg
i have tested this on some txt files to fle
C:\Documents and Settings\Owner\test>ren *.fle *.txt
Volume in drive C is Hard Drive
Volume Serial Number is 5006-5177
Directory of C:\Documents and Settings\Owner\test
07/25/2009 08:06 AM <DIR> .
07/25/2009 08:06 AM <DIR> ..
07/25/2009 08:03 AM 4 blarg1.txt
07/25/2009 08:03 AM 4 blarg2.txt
07/25/2009 08:03 AM 4 blarg3.txt
07/25/2009 08:03 AM 4 blarg4.txt
4 File(s) 16 bytes
2 Dir(s) 23,233,605,632 bytes free