Fist off, since this thread is much more technical than it is "Hello, world" in nature, I'm going to move it from the New Users and Introductions to the MS-SQL forum.
Next, there are multiple ways to check the size of the databases. Assuming that you are running SQL 2000, probably the easiest way is to open SQL Enterprise Manager, in the navigation pane (leftmost on most systems) you'll find a tree control... Pick your server, then databases, then the database that interests you (such as msdb or tempdb). Right click, and select Properites from the menu. You'll see tabs for data and log files.
If you need to "clean house" because of a log file being full, close the properties dialog, right click again, select All Tasks, then Shrink Database.
Refer to BOoks online for this error, that has complete information to recover it. I would also suggest to check what kind of operation is making this alerts raised and add more disks to accomodate the resource intensive operations.