PicBackMan is the best tool when it comes to migrating photos from one online account / service to another online account or your Windows PC. PicBackMan is an automatic bulk photo uploader and a power packed tool that allows you to de-dupe your online accounts as well as migrate large photo collections to a different account.
Here's how to migrate photos from Picasa to your Windows PC :
1. Associate and activate your Picasa account.
2. Go to MIGRATE tab and click on MIGRATE ACCOUNT button. Click the check box next to Picasa and then click SAVE.
3. Now click on CHECK BOX corresponding to 'My Computer'.
4. Now click on MIGRATE button.
5. Now all your Picasa photos will be migrated to your Windows PC.
Note that PHOTO and VIDEO upload shouldn't be running. Also it might take time for migration to complete because it depends on number of photos you have in your Picasa account.
You can install PicBackMan's Picasa uploader for Mac from the website and start backing up photos and videos.