APK Files Giving Parse Error?

I have recently gotten back into developing Android applications. However, every time I try to run an application on my phone now, I get the error message - "Parse Error - There is a problem parsing the package."

Two years ago, I used Eclipse to develop projects, which is what I'm using now. Though, I had to reinstall eclipse and everything else. So it's a new setup but I believe it's the same general setup.

I can put my old .apk files on my phone, and they work fine. If I put any newly created .apk files on my phone, I get the parse error.

I also found that if I compile any OLD projects and then copy those onto my phone, that I get the parse error.

This leads me to believe I've got something set up wrong in Eclipse that my phone isn't liking.

I thought maybe it was the virtual device manager, so I've tried several combinations of virtual devices - 1.5, 2.1, 2.2, 2.3.3 - using any of them still gives the parse error when trying to run the applications on my phone.

By the way, all of these projects work in the emulator running through Eclipse, so I know they work.