Looks like your iTunes db is holding a reference to some files that are no longer there. This happened to me once after I moved my iTunes library to another account.
Close down iTunes (so that the iTunes Music Library.xml file is updated), then open iTunes Music Library.xml in a text editor. Look for the podcasts there and take a note of their location.
Try to recreate the path to them - for instance, if they're in /Users/oldAccount/Music/iTunes/iTunes Music/Podcasts, recreate that folder (of course, you moved from a Dell, so the path is likely c:/something) -and copy some random mp3s to it, renaming them same as the ones the xml file refers to.
Then they'll show up in iTunes and you can delete them.
However ... did you move from the Dell to a Mac? I'm not sure if this would work in that case.