It's in ~/Library/Widgets
Apple hides the users Library folder.....you can quickly get to it in Finder by holding Option when clicking on the Go menu (with Finder as the active app). You'll see the link for Library.
That's not what fingerman was talking about. I had iStat Pro widget installed in my dashboard since who knows when, but when I looked at my /Library/Widgets folder where all the default widgets are, that's all it contained - default widgets. I had no idea where the package for iStat Pro widget was on my HDD. Spotlight couldn't find it. This was fingerman's dilemma, too.
In the end I just removed the iStat Pro widget from the dashboard, then removed it from the list of widgets (click + in dashboard, then click - to make the widgets wiggle, then click x on iStat Pro's icon). I assume this also removed the package file, wherever it was.
I then downloaded iStat Pro again, moved the package file into the /Library/Widgets folder, did the changes suggested in this thread and now have iStat Pro back. With processes.
Thanks a ton for that tip Buduls! (and the nicer icon, zepman)