Binos,
The first thing to try is repairing your disk permissions.
1. Open Disk Utiliy (Utilities Folder, within the Applications folder)
2. Click the top disk on the left (your startup disk)
3. Click "Verify Permissions"
4. Click "Repair Permission"
5. Quit Disk Utility
This may sort it out - it's the only bit of disk management you need to do on the Mac, usually once a month or when you install new software.
Another classic "slowing down" reason is because the disk is too full. You need to keep 3Gb-5Gb of the disk free for OSX to run well.
Try Disk Utility, if that doesn't work, perhaps you could let us know more details about your system to work out other protential reasons why it's running slowly?
Applemacster