I don't use Picasa, so I can't give you a direct answer.
However, I'm somewhat confused as to how you're planning to get your negatives into Picasa. You can't use a basic flat-bed scanner because scanning negatives involves using transmitted light, not reflected. So I assume that you've got a dedicated film scanner or (more likely) a flat-bed scanner with a built-in film scanning facility. If so, the TWAIN driver for the scanner, which imports the image into Picasa, should have a 'negative' or 'reversal' setting, so that the image automatically appears in its 'positive' form, as soon as it's scanned in.
If, for some strange reason, your scanner doesn't allow you to reverse images, I'd be surprised if Picasa doesn't offer such a facility. It's standard to almost all image-processing software, including the wonderful IrfanView (which, in my opinion, should be on everyone's PC):
http://www.irfanview.com/
(With IrfanView, image reversal is achieved simply by going to Image > Negative).
Chris