Download and burn the Ultimate Boot CD and then boot from it. If it boots, run the Windows Memory Diagnostic test and then run the specific manufacturer's hard disk diagnostic utility. You can usually look in the BIOS to determine what brand hard drive you have.
If your hard drive doesn't have a utility (Toshiba comes to mind) you MAY have a diagnostic partition on the computer that you can boot up to. Press F12 for Dell and see if it lists something like Diagnostics or Hard Drive Diagnostics. It could be a whole suite of diagnostics or JUST a hard drive test.
If just the hard drive, run that. If a whole list of items can be tested - run the EXTENDED test and see if anything fails.