Disk Utility: invalid volume file count & directory count

My MacBook OS X version 10.5.8 has ran quite slowly occasionally but I think that's due to the fact I've had it for almost 5 years. But then yesterday it kept on freezing every few seconds so that it was impossible to do anything. I then left it overnight and in the morning cleared the safari cache and kept rebooting it which eventually stopped it freezing so often. But now certain applications such as iTunes keep quitting before they even open properly and applications that do work still freeze every so often. So then I ran disk utility and verified the disk which came up with this:

"Performing live verification.

Checking Journaled HFS Plus volume.

Checking Extents Overflow file.

Checking Catalog file.

Checking multi-linked files.

Checking Catalog hierarchy.

Checking Extended Attributes file.

Checking volume bitmap.

Checking volume information.

Invalid volume file count

(It should be 759557 instead of 759584)

Invalid volume directory count

(It should be 157930 instead of 157903)

The volume Macintosh HD needs to be repaired.

%)

Checking multi-linked files.

Checking Catalog hierarchy.

Checking Extended Attributes file.

Checking volume bitmap.

Checking volume information.

Error: Filesystem verify or repair failed."

However the 'repair disk' button is dimmed so I can't click on it. Then when I typed in my problem on the help bar it said I need to use the installation disk to be able to repair this but I don't know where this disc is, so was wondering if there are any ways of fixing this without needing a disc?