The application DGAgent quit unexpectedly -- repeatedly pops up every time i "ignore", "report" or "relaunch"

The error you're getting from Skype is because you're not using an Intel-based Mac. Skype only runs now on Intel-based Macs so I assume you're running a PowerPC iMac G5 or something are you? You also need to be running Leopard/Snow Leopard. Here's a screenshot of the System Requirements on the Skype Download page:

So, the Skype issue is not related to the DGAgent message.

As regards the startup of DGAgent on login, you can try the following:

1. Open System Preferences
2. Click Accounts
3. Select your account
4. Click Login Items
5. Highlight Drive Genius if it's listed and click the '-' button on the lower left to remove it.

This will prevent OS X from attempting to start it on login. See screenshot below.