I have been using Limewire with very little problem until a couple of weeks ago. Now I find that I canot preview or view MPEG/MPG files. After loading Quicktime, I can see avi files & mov files.

I have uninstalled Realplayer & Windows Media Player & reinstalled them (WMP 11) - but the Realplayer still returns a 'General' error message & WMP an error message which seems to be related to my sound card - yet music files etc. play fine.

This seems to be the opposite of what other people are experiencing.

Any ideas??

Might be a codecs issue....
You could try to use a different media player such as the free VLC player with built-in codecs which can play almost all media files without problems. You can download it from here:
http://www.videolan.org/vlc/
As a side note, to preview files with it you'll have to open the files directly from the Incomplete folder and not via the LW preview function but make sure to NOT allow VLC to make any changes on the incomplete file when previewing.

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Thank you. I seem to have downloaded VLC but when I click on either the complete or incomplete file - initially it was still defaulting to Realplayer. I then amended Realplayer so it was not the default player for mpeg / mpg files - but I can't see how to make vlc play the files.

See Setting Default video app. You'll need to apply the same principle for both avi, mpeg/mpg, etc. files.
For previewing, I'd recommend going to the LW Library window, select incomplete folder, find & select the file & press Launch. VLC on windows has problems with preview function in the newest vlc release. Also this will stop LW creating a PREVIEW file & storing it in your incomplete folder. Preview files can take up a lot of space. Oh ... duh ... Foolofthehill already said that ... sorry.