My system keeps freezing. how do I run disc repair?

Question

Processor  2.16 GHz Intel Core 2 Duo
Memory     1GB 667 MHz DDR2 SDRAM
Startup Disk   Macintosh HD
  Model Name:          iMac
  Model Identifier:          iMac5,1
  Processor Name:          Intel Core 2 Duo
  Processor Speed:          2.16 GHz
  Number Of Processors:          1
  Total Number Of Cores:          2
  L2 Cache:          4 MB
  Memory:          1 GB
  Bus Speed:          667 MHz
  Boot ROM Version:          IM51.0090.B09
  SMC Version (system):          1.9f4
  Serial Number (system):          W86360WZWRX
  Hardware UUID:          00000000-0000-1000-8000-0016CB9EA9DB
I keep getting a line of distorted pixels across the screen and sometimes a distorted portion of the page, which clears or appears in another area when I move the cursor over it. Often times my whole page freezes and I am left to the spinning rainbow and am forced to shut down with the off button.

Answer 1

Repair the Hard Drive and Permissions
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. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list.  In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive.  If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the installer.
If DU reports errors it cannot fix, then you will need Disk Warrior and/or Tech Tool Pro to repair the drive. If you don't have either of them or if neither of them can fix the drive, then you will need to reformat the drive and reinstall OS X.