In the MS Office case, just trash the folder. That will trash everything inside the folder as well, including the apps. Same thing goes for other Applications which may be in their own folders (for instance, the iWork '06 trial).
If you want to reclaim some space without reinstalling, you can do a couple things. One would be to trash some of the printer drivers located in /Library/Printers. I would not recommend this until you are more familiar with the Operating System, and you feel safe with such an operation.
Also, you can remove many of the language files with various third party utilities. My favorite is
Monolingual. Run that utility, and it will recover hundreds of megabytes from a default installation of OS X. Just make sure to read the instructions beforehand.
Welcome to the world of OS X. I'm sure you'll feel more comfortable with it soon.