Freeing up disk space

I am a newbie in administering systems (as I am a developer), but lately, I am doing basic admin tasks on windows servers. We have an automated process that restores a database that failed with the message:

There is insufficient free space on disk volume 'c:\' to create the database. The database requires 2218459136 additional free bytes, while only 1219432448 bytes are available.

When I looked at C: drive -> properties, it says the free space is 3GB. Why this discrepency? (As per the message, the database needs 2.1 GB)

I guess I have to free up some space. I checked for any files that can be deleted and didnt find any. As a temporary solution, should I do a defrag on the system (Would that screw up anything)?

Also, is there a way to find out which folders are taking most of the disk space?

Thanks

EDIT: Also, I read about Disk cleanup utility. Is that risky?

Not to throw more software suggestions around, but I use windirstat (http://windirstat.info/) to find files that are taking up too much space.

If your system drive is down to 3gb then you have much more serious problems than trying to restore a database.

Also, is there a way to find out which folders are taking most of the disk space?

TreeSize Free is your handy little helper here.

_{(source: jam-software.com)}

Defragging won't screw anything up, but probably won't help the disk space either. Clear out the temporary directory, but also dig around the file structure for "temporary" data and downloads and service-packs and - if you're seriously short on space and willing to befall irony in the future - system restore points.

(Back things up before you do any of this. If you're thinking of deleting something you don't recognise, rename it first and see if anything notices that it's gone)

However, if you're at 3GB remaining and your DB needs 2GB, now's a wonderful time to consider investing in some more hard-drive space.

A defrag will not harm your system or free up disk space.