I think GIMP supports reading and editing of .psd documents.
I use GIMP on my Mac and its also on my Windows Vista laptop (it's how I drew my avatar - although that's not really selling it!). I quite like it, but it does take a little time to learn (much like photoshop).
Yes, the GIMP will handle PSD files admirably. If it's a bit complex for what you need, try IrfanView (which will read, edit and export to other formats) - it's freeware but it's been around for years and is well-respected.
Mike
The secret of life is to aim below the head (With thanks to MMM)