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:


Open System Preferences
Click Accounts
Select your account
Click Login Items
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.