This isn't really a question, but a solution I thought I'd share in case anyone else runs into the same problem I just did.
The scenario: I've got a retail copy of ME2. It installed fine the first time around, but I decided to reinstall it because it crashed when starting the Migrant Fleet mission to help Tali. Unfortunately I got an error message each time I tried: Installation Failed: A Data Error Occured.
The solution: Apparently this is caused by corrupt .rar files. I finally managed to fix this as follows:
1) Copy the contents of both DVDs to a folder on your harddrive, e.g. "C:\Mass Effect 2". Some files on DVD2 will be the same as on DVD1, you don't need to replace those.
2) Run Setup.exe from your harddrive. In my example that would be the file "C:\Mass Effect 2\Setup.exe".
3) Pay close attention to the installer. Chances are that you'll run into the Data Error sooner or later. Fortunately the installer tells you which .rar it is currently unpacking. Delete that .rar and copy it again from your DVD. Hopefully this time around, it won't be corrupt and this time the installer will continue past this point.
4) After I finally got the installer to finish, Mass Effect 2 didn't run yet, but fortunately it did after running the 1.02 patch.
If you have WinRAR installed, you can even replace the file in a corrupt .rar that is causing the problems. Like I said the installer tells you which .rar it is unpacking and it even tells you which file it's currently working on. If that file is corrupt, the installer will apparently freeze for a second or so before giving the Data Error. If you make a screenshot (or have photographic memory ) you can replace the corrupt file by opening the .rar on both your harddrive and your DVD (which is what you need WinRAR for, or a similar program) and copy/paste the file from the .rar on your DVD to the .rar on your harddrive.
It could be that there is an error log somewhere that tells you which .rar and file the installer crashed on. If so, feel free to reply with information on where to find that. It could save someone the hassle of paying close attention to the installer and making screenshots every now and then.
This solution is tedious: it cost me somewhere between 5 and 10 tries to fix all the corrupt .rars. The biggest culprits where movies.rar and other.rar. But ultimately it worked and ME2 is too great a game to pass up that opportunity!
I hope this helps someone who runs into the same problem.