Problems with PSD files

I was wondering if someone could help me?

I just finished a contest and the client is having problems opening the PSD files I sent them.

I designed the buttons in Photoshop 7 and the client is trying to open the files in Photoshop 5. Could this be the problem? OR maybe it’s something that I did wrong?

PM and I can give you the link to the files.

Thanks
Heather