changing file name extensions

[#1]

I Have been trying to rename some MP3 files where the tracks are showing as track one track two etc, however when I try to change the name of the file I get the message "File may become unusable".
How do I get round this ?
I Recall that at one time I had no problem in renaming tracks of mp3 files.
Please advise.
Using windows XP
NB the files are indeed unusable as they fail to play in an mp3 player.

[#2]

when you change a file name such as "track1.mp3" you need to change it to "newtrack.mp3"
Note the .mp3 at the end.
However using Windows Explorer the default setting is to hide known file extensions so "track1.mp3" might appear as "track1" so changing it to "newtrack.mp3" would in fact change it to "newtrack.mp3.mp3" which would rather mess things up and so you would just change the name to "newtrack"

[#3]

@Oldjim - give the OP is getting the warning - its pretty obvious that extensions are visible.
When they are hidden, you can't edit them - thus the message would not appear, and the issue would not exist.

[#4]

When they are hidden you can still type in the extension
I just tried and when the extensions were visible again I had this track1.mp3.wav
However changing a .mp3 to a .mp3.mp3  didn't make it unreadable and the message didn't appear 

[#5]

You are wrong.
You can type in the filename when the extension is hidden - you cannot type the extension in.
In your example, you have a file called track1.mp3 with a file extension of .wav
Full stops are permitted in filenames.

[#6]

see edit

[#7]

Why hide the extensions when the Windows system relies on the extension to decide on which program to run?
If it just looked at the first few bytes of the file it would know that it's a mp3 file!

[#8]

Don't ask us - ask Microsoft as it is their silly default

[#9]

Slightly OT, but if you've got a lot of files to rename try:
http://www.bulkrenameutility.co.uk/Main_Intro.php
Works a treat, very powerful but takes a bit of getting used to.
Regards,
Neil

[#10]

you can also use http://www.freecommander.com/ as a replacement for Explorer, multi pane, change a lot in bulk

[#11]

Quote from: Oldjim

When they are hidden you can still type in the extension I just tried and when the extensions were visible again I had this track1.mp3.wav However changing a .mp3 to a .mp3.mp3  didn't make it unreadable and the message didn't appear

Nice work - change your post when somebody tells you it's wrong.
Shame you still don't get it.
You haven't changed the extension, it is still .mp3 - just because the filename also contains .mp3 does not change anything at all.
track1.mp3.mp3 is a track called track1.mp3 with an extension of .mp3

[#12]

If you look at the timing they were done at exactly the same time. I was making the edit while you were posting.
Perhaps if you stopped to check before you posted it would be better

[#13]

Thanks for the responses.... by changing the filename and ending it with mp3 as suggested by Oldjim the files work fine so the problem is solved.
The additional information supplied by Pierre-Pierre & Njal regarding the bulk change of file names was also very useful.