Im getting this message when I check my mod list in the launcher...

then when I load my game I see "morrowind.esm is not a valid TES file"

Are my saves corrupted?

Is the game installed in Program Files or Program Files(x86)?

If it is, then move it. Luckily, in this game, as long as you used the installer to install it, then all you have to do is move the folder in Windows Explorer. Something like C:\Games\Morrowind or what ever as long as it is not in a UAC controlled directory. Make a new shortcut to the launcher in the new location. Just note, some mod tools may think it is still in the original install directory, so make adjustments as needed.

Its in Program Files(x86), but why has this error only now just appeared? Ive been playing the game fine for weeks and thats where its been installed.

Probably because you have been lucky up until now, or you have disabled UAC on your machine (not recommended)

Even if this is not the root cause of this issue, the more you get into mods, the more the install location is going to give you problems. Just move the game, it's real easy in this case.

Nope.. moved it, im still getting the error. This only happened since using the 4gb patch.. could that be the cause?

*edit... from what I can see.. my MW.esm seems to have dissapeared.. in place is a 'MW.esm backup' I dont know where that has come from.

Ok ive managed to fix it by simply copying a backup of the MW.esm... no idea what happened to it. Can a CTD on exit cause the MW esm to corrupt?