Boot up from your Snow Leopard Install DVD while holding down the "c" key.
Select the language you wish to use, and then choose "Disk Utility" from the menu bar.
Your HD should appear in a panel on the left hand side of the window which opens. Select it and then click on the "repair disk" button in the right hand section of the window.
If Disk First aid is unable to complete the repair you will need to use a stronger 3rd party utility like DiskWarrior or TechTool Pro.
Once this process has been completed restart your computer.
First, there will always be repeating Permssions messages. You can ignore those and stop repairing Permissions; it's not needed.
Two ways of dealing with the slowdowns. I'm not seeing any excessive paging in your System Memory screenshot, but it might help anyway to run fewer programs concurrently, if possible. Other than that, install Developer/XCode from optional installs on the install DVD. When things slow down, you will be able to run purge (from Terminal), which will clear the virtual memory being used. But this will only be a temporary fix, until the VM starts accumulating again.