Install or Reinstall Snow Leopard from Scratch
If possible backup your files to an external drive or second internal drive.
1. Boot from your Snow Leopard Installer Disc. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities menu.
If you are preparing an external or a non-startup drive, then open Disk Utility in your Utilities folder.
2. After DU loads select your hard drive (this is the entry with the mfgr.'s ID and size) from the left side list. Note the SMART status of the drive in DU's status area. If it does not say "Verified" then the drive is failing or has failed and will need replacing. SMART info will not be reported on external drives. Otherwise, click on the Partition tab in the DU main window.
3. Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Click on the Options button, set the partition scheme to GUID then click on the OK button. Set the format type to Mac OS Extended (Journaled.) Click on the Partition button and wait until the process has completed.
4. Quit DU and return to the installer. Install Snow Leopard.
5. Download and install the Mac OS X 10.6.8 Update Combo v1.1 updater.
6. Reinstall your third-party applications and restore you data.
The build up of stuff might just be dust. You can try cleaning it by cautiously vacuuming the exhaust port - use a soft brush attachment. Do not blow dust back into the MacBook.
The following are some troubleshooting steps you can take to determine if you have installed software that is burdening your computer. Do not succumb to installing some utility or tool advertised to perform some magical incantation that will make it run like new. In nearly every case they will do the exact opposite. If you already did this you found the problem.
General Mac maintenance: Tips to keep your Mac in top form
General purpose Mac troubleshooting guide: Isolating issues in Mac OS X
Creating a temporary user to isolate user-specific problems: Isolating an issue by using another user account
Identifying resource hogs and other tips: Runaway applications can shorten battery runtime