Picasa doesn't store your photo's. It just indexes them. They are somewhere on your hard drive.
Windows Photo Gallery lets you "find, edit and organize your pictures". I'd start with finding them.

- http://windows.microsoft.com/en-US/windows-vista/Open-Windows-Photo-Gallery tells how to run the program. Isn't it nice that Microsoft provides such elementary help?
- http://windows.microsoft.com/en-US/windows-vista/Find-a-picture-on-your-computer tells how to find pictures.
- http://windows.microsoft.com/en-US/windows-vista/Windows-Photo-Gallery-frequently-asked-questions has other useful info.