...with (Vista) thank you? Sorry to ask a silly question but a great friend has sent me a .dat file. I think it's a music file. I can download and save it but not open & play it. Any suggestions welcome. I'm on Vista. Thanks in advance

A .dat file? Loads of programmes create those, ask your friend to send again as mp3, as there is no way to easily open it.

To solve this really quickly, download VLC player and see if it opens the file. If it doesn't, rename it to .mp3 or .wma and see if it works. However it doesn't guarantee that you will be able to play it.
Download VLC player from here:

http://www.videolan.org/vlc/

If it still doesn't, try this. First you might need to find out what file it really is. Since you said it's a music file, you might need a program that can analyze what codec it has. This program is called gspot. I know, bad name. But you can find it here.

http://www.headbands.com/gspot/v26x/index.htm

Open the program and drag the .dat file into it. It should show its extension. After that, you can rename the .dat file to whatever codec it really is.

VLC should be able to play the correctly named file.