Majin is right to a certain extent. As far as Microsoft are concerned, you're stuffed.
However, there are two options available to you, neither of which will have guaranteed results and neither of which will be able to put your hard drive back to exactly how it was before. When you do a quick format on a hard drive, all that it does is marks every sector of the hard disk as free to be overwritten, although the data is actually there but it is hidden from the user. This is because the File Allocation Table (or the NTFS equivalent in Windows 2000/XP) is removed or re-written with an 'empty' version. Now, to be honest, I'm not sure whether the Windows ME install actually does a 'quick format' as we know it in Windows 2000/XP or whether it does a 'full format'. I know that the Windows 98 format process takes MUCH longer and I think that Windows ME is the same. Therefore, I assume that this is actually doing a 'full format', whereby it writes zeros into every data sector.
If anybody knows that the above is incorrect, please do correct me as I'm not entirely sure.
Now, if you've got a hard disk with all the data still there but the file allocation table wiped, then you have two options:
1. Get some data recovery software.
2. Send the hard disk off to a data recovery specialist.
The second option will always have better results but expect to pay £500 or more. If your sister is like most users, then she probably won't consider her data to be worth spending £500 and would probably prefer that you try to run some data recovery software on it.
BEAR IN MIND that, if you run data recovery software on the disk, then it is far less likely that a data recovery specialist would be able to recover more data from it. This is because the recovery software will change the contents of the disk during its recovery tasks and will probably render it unuseable after it has finished. If it gets most of your data back, great. If it says that it can't, then the disk is as good as in the bin.
If your sister considers her data to be worth enough to pay a data recovery specialist to try to get it back, then do NOT run any software on it yourself.
Other things to bear in mind are that you have actually installed more new files onto the hard disk after formatting it. This means that some of the data that was previously on the disk but just hidden could now have been overwritten permanently by the Windows ME installation.
Also, data recovery results are never guaranteed.
If you want to go down the software route, here's some of what I found on a Google search:
http://pcperformancetools.com/recovermyfiles.htm (This is a link to a comprehensive article about data recovery, with a link to RecoverMyFiles, one of the most well-known recovery programs)
http://www.active-undelete.com/ (This is another relatively well-known - or at least well-advertised - program)
http://www.file-saver.com/undelete/?pu=false&srec=true http://www.powerdatarecovery.com/?gc...FRcGEgod4Ad_og
PLEASE NOTE that the above links and information are NOT recommendations by myself or by D-A-L.com or its owners. The comments above are merely my own views and opinions, which we hope will assist you in making your own decision.
Despite all the disclaimers, I hope all that helps! ;-)