Moving your catalog could be a good thing.
Although your catalog is just a brain, and not a bucket, it can still grow to be a fairly large size. Moving it off of your startup drive and onto another drive, even an external one will free up space for your operating system.
Make sure you have quit out of Lightroom.
Find your catalog on the hard drive and then copy the entire Catalog folder to the new location.
After copying the catalog, double click on the copied Lightroom catalog file to open Lightroom.
Go to your Lightroom preferences to set which catalog Lightroom should open automatically. On a PC you will find Catalog An application's behaviors and preferences are controlled with settings. More under the Edit tab. On a Mac you find it under the Lightroom tab. By default Lightroom automatically opens the most recently used catalog. If you have multiple catalogs, you can choose a specific catalog in this drop down list.
Delete the original catalog in your pictures folder on your startup drive.