According to your screenshot, you have more than enough free space on the drive (254GB), so I'm going to assume that you have a 320GB hard disk, and are concerned about why the system is only showing 254GB free of 298GB?
There are various explanations, but the most common is because drive manufacturers count drive capacities in decimal units of 1000MB (1GB), but Windows (and every other OS in existance) uses binary units of 1024MB (1GB), and this is where the imbalance comes from.
What also affects total reported capacity are various houskeeping allocations on the hard disk, such as partition tables, boot records, file systems, etc.
The following table shows what my own hard disks are reportng:
|Manufacturer Capacity||Actual Capacity |
You shouldn't be too concerned over this, as this is perfectly normal.