As others have said, have a look at The Gimp. That is very powerful, but may present a steep learning curve. Another free alternative is IrfanView - which also comes with IrfanView Thumbnails, which can display your entire photo library as thumbnails - making it easy to find stuff when all the files have names like DSCFnnnn.JPG
Which version of W7 are you running? Are you *sure* that your Photoshop software won't run? I've got Photoshop 5 LE - which is years old, and came bundled with something or other (camera, scanner - can't remember) and that works ok under W7.
I've got W7 Professional 32-bit, and a hell of a lot of stuff runs on that which isn't officially supposed to - including some very old stuff like Quicken 98. Even old hardware will sometimes work if you experiment with drivers. For example, I've got a USB-based MIDI interface for which there's no W7 driver. It wouldn't work with the XP driver, so I tried the driver for W2000, and bingo! When installing old software, you sometimes get some funny messages appearing. Unless they're fatal, just ignore them, and tell it to carry on!
I've got one or two programmes which work under W7 but do slightly funny things[1] - so I have to run them in a virtual XP machine inside W7 to overcome these quirks.
[1] Quicken 98 works, but can't create invoices. Capella Scan 5 (music OCR software) works but the menus are all in German! Finale 2003 (music notation software) works but when you print multiple copies, you get the *square* of the number you specify - 4, 9, 16 for 2, 3, 4 etc.